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Difference between State and props in ReactJs

In this article, we will discuss the states and props in ReactJS. Props and state both provide the details about the item but have differences. Let’s take a look at how to pass props into the component and how to update a component’s state.What is State?The status of a react class component instance can be described as an object of a collection of observed features that control the component’s behavior. In other words, the component’s state is an entity with some details that can alter during the component’s lifetime.This data is kept inside a component. This particular aspect is local or owned. The component itself uses the setStatus function to update the state. The state is an updated structure that is used to contain and can modify data or information about the component over time. The state change may occur as a user activity or device event response. It decides the actions of the component and how it is to be done by the core of the React component. As easily as possible, a state should be held. It is the local state or details of the part. Only in the component or by the component directly can it be accessed or changed. Components of react have a built-in state object. The state object is where the property values belonging to the item are stored. The component is restored when the state object changes.What are Props?React is a library focused on components that separate the user interface into reusable little bits. These components have to transmit (send data to each other) in some cases, and the way to transmit data among components is through props. “Props” is a special React keyword for proprietary purposes, used for data transmission from component to component. But the key part of this is the transmission of data with props in a uniform flow. (parent-to-child route)In addition, props data are read-only, meaning that parent data cannot be modified by child elements.Difference between State and PropsReact is an opensource JavaScript library that offers a visual overview of the JavaScript architecture for conventional MVC. React promises programmers a model in which substrates cannot directly influence enclosing components—data are downstream, data changes in HTML are efficiently modified, and the DOM is abstracted to boost performance using Virtual DOM.How can we access data from the previous component if the data is just being flowed from components to components? The answer is props. React uses data transmission props that we need to transfer to various components. The difference comes in which component the data are owned. State is owned locally and the component itself updates it. Props are owned and read-only by a parent. Props can be changed only if an upstream shift is caused by a callback function passed on to the child.A prop can be passed on to the child in the state of a parent. They apply to the same value but can only be updated by the parent variable.How are props passed into the component?We can pass props to any component by declaring HTML tag attributes.<DemoComponent sampleProp = "HelloProp" />We transfer a sampleProp in the above code snippet to the DemoComponent part. This prop has the ‘HelloProp’ value. See how we can access this advice now.In the class components to which the props are transferred, we can access any props.this.props.propName;We can use the above syntax to access any prop from within a component class. The “this.props” is a type of total object that stores all props from an item. The propname, which is the propName, is the key.Passing information from one component to other:This is one of React’s coolest characteristics. We should make it possible for components to communicate. To understand this, we will look at two components Parent and Child. We will pass information to the Child component as advice from our parent component. We can offer a part as much advice as we want. The content of a prop is not permitted to be changed. No matter what kind of component it is, nobody is allowed to change their advice, whether functional or class-based. The difference comes in which component the data are owned. State is owned locally and the component itself updates it. Props are owned and read-only by a parent. Props can be changed only if an upstream shift is caused by a callback function passed on to the child. A prop can be passed on to the child in the state of a parent. They apply to the same value but can only be updated by the parent variable.How do you update a component’s state?Although a react component can have an initial state, the actual power is in updating the state — the component should not be in either state if we don’t have to update the state. State is reserved only for data that changes our part and can be seen in the user interface.We use this.setState() instead of changing the state directly using this.state (). This is a feature for all components that use state, and allows us to inform React that the state of the component has changed. This way the component knows that it can return because its status has changed and the user interface will probably change as well. It is very efficient to use a setter function like that. React intentionally waits until all components in their event handlers call setState() before they start returning. This increases efficiency by preventing excessive re-renders. You may also ask why React does not update this, however. Two major reasons exist: The consistency of props and the state is broken, which causes problems that are very difficult to debug. This will make it difficult to introduce such new features. React will load several setState() calls for performance into a single update. Due to the asynchronous of this.props and this.state, you cannot depend on their values for the next state to be calculated. To fix it, use a second setState() form, which accepts a function instead of an object. This function is the first argument for the previous state, and the props are the second argument when the update is applied: this.setState(function(state, props) {   return {     counter: state.counter + props.increment   }; });Is state changeable?A state change takes place on the basis of the user input, which triggers an occurrence. React (with status) components are often made on the basis of state data. The initial knowledge is held by the State.Thus when the state changes, React will be notified and the DOM will be re-rendered immediately; not the whole DOM but only the modified portion. This is one of the reasons for the fast reaction.And how do you notify React? You thought: with setState ( ). The setState() method triggers the mechanism for rendering the modified components. React is notified, knows which part(s) to alter, and does so quickly without restoring the entire DOM.Is state created in the component?Let’s see the constructor method:constructor() {   super();   this.state = {     count: 0,   }; }This is where the state gets the initial data. The initial data (as above) can be hard-coded, but it can also come from props.However, it makes sense – you cannot adjust props but the data a component receives wants to do so. This is where the state enters.Component typesStateless component — Just props, no state.  Besides the render() function, there’s not much going on and all its logic is about the props that they get. This makes it easy to track them (and test for that matter).  The stately component — state as well as props. These are also called state managers. They are responsible for communication between clients and their servers (XHR, Web sockets, etc.), data processing, and user events.What happens when state changes?React Components allow you to break the UI into separate, reusable components so that you can look into every single item on an isolated basis.Components are conceptually like functions in JavaScript. They accept arbitrary inputs and return elements of react that describe what should be shown on the screen. If you have to allow the user to enter something or to alter the variables that the component is supported by, you would have to setState.State allows React components in response to user behavior, network responses, and everything else to adjust their performance over time, without violating this rule. Class-defined components provide additional functionality. Local status is the only class component function available.Can I use state in every component?In the early days, only class components, and not functional components, were included.That’s why stateless components are known for their functional components. However, state can now be used in both class and functional components following the implementation of React Hooks.You can only use status in class components if your project does not use React Hooks.The component State and Props share some common similaritiesProps and states both provide details about the item, but they are used differently and must be kept separate.ConclusionState refers to the component’s local status which cannot be used and changed outside the component and can only be used and modified within the component. On the other hand, it provides reusable components by enabling components to obtain data in the form of props from the parent component. We may change the state of a component with setState. These notifications are also triggered by events. setState is called asynchronous and merged with every entity in the current state. We may also transfer a setState function to allow us to write status changes based on the current status values. Most of your components can be stateless when you create an app. Props transfer parent-to-child data. They are unchangeable and are thus unchanged.

Difference between State and props in ReactJs

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Difference between State and props in ReactJs

In this article, we will discuss the states and props in ReactJS. Props and state both provide the details about the item but have differences. Let’s take a look at how to pass props into the component and how to update a component’s state.

What is State?

The status of a react class component instance can be described as an object of a collection of observed features that control the component’s behavior. In other words, the component’s state is an entity with some details that can alter during the component’s lifetime.

This data is kept inside a component. This particular aspect is local or owned. The component itself uses the setStatus function to update the state. 

The state is an updated structure that is used to contain and can modify data or information about the component over time. The state change may occur as a user activity or device event response. It decides the actions of the component and how it is to be done by the core of the React component. 

As easily as possible, a state should be held. It is the local state or details of the part. Only in the component or by the component directly can it be accessed or changed. 

Components of react have a built-in state object. The state object is where the property values belonging to the item are stored. The component is restored when the state object changes.

What are Props?

React is a library focused on components that separate the user interface into reusable little bits. These components have to transmit (send data to each other) in some cases, and the way to transmit data among components is through props. 

“Props” is a special React keyword for proprietary purposes, used for data transmission from component to component. 

But the key part of this is the transmission of data with props in a uniform flow. (parent-to-child route)

In addition, props data are read-only, meaning that parent data cannot be modified by child elements.

Difference between State and Props

React is an opensource JavaScript library that offers a visual overview of the JavaScript architecture for conventional MVC. React promises programmers a model in which substrates cannot directly influence enclosing components—data are downstream, data changes in HTML are efficiently modified, and the DOM is abstracted to boost performance using Virtual DOM.

How can we access data from the previous component if the data is just being flowed from components to components? The answer is props. React uses data transmission props that we need to transfer to various components. 

The difference comes in which component the data are owned. State is owned locally and the component itself updates it. Props are owned and read-only by a parent. Props can be changed only if an upstream shift is caused by a callback function passed on to the child.

A prop can be passed on to the child in the state of a parent. They apply to the same value but can only be updated by the parent variable.

How are props passed into the component?

We can pass props to any component by declaring HTML tag attributes.

<DemoComponent sampleProp = "HelloProp" />

We transfer a sampleProp in the above code snippet to the DemoComponent part. This prop has the ‘HelloProp’ value. See how we can access this advice now.

In the class components to which the props are transferred, we can access any props.

this.props.propName;

We can use the above syntax to access any prop from within a component class. The “this.props” is a type of total object that stores all props from an item. The propname, which is the propName, is the key.

Passing information from one component to other:

This is one of React’s coolest characteristics. We should make it possible for components to communicate. To understand this, we will look at two components Parent and Child. We will pass information to the Child component as advice from our parent component. We can offer a part as much advice as we want. 

The content of a prop is not permitted to be changed. No matter what kind of component it is, nobody is allowed to change their advice, whether functional or class-based. 

The difference comes in which component the data are owned. State is owned locally and the component itself updates it. Props are owned and read-only by a parent. Props can be changed only if an upstream shift is caused by a callback function passed on to the child. 

A prop can be passed on to the child in the state of a parent. They apply to the same value but can only be updated by the parent variable.

How do you update a component’s state?

Although a react component can have an initial state, the actual power is in updating the state — the component should not be in either state if we don’t have to update the state. State is reserved only for data that changes our part and can be seen in the user interface.

We use this.setState() instead of changing the state directly using this.state (). This is a feature for all components that use state, and allows us to inform React that the state of the component has changed. This way the component knows that it can return because its status has changed and the user interface will probably change as well. It is very efficient to use a setter function like that. 

React intentionally waits until all components in their event handlers call setState() before they start returning. This increases efficiency by preventing excessive re-renders. 

You may also ask why React does not update this, however. 

Two major reasons exist: 

  • The consistency of props and the state is broken, which causes problems that are very difficult to debug. 
  • This will make it difficult to introduce such new features. 
  • React will load several setState() calls for performance into a single update. 
  • Due to the asynchronous of this.props and this.state, you cannot depend on their values for the next state to be calculated. 

To fix it, use a second setState() form, which accepts a function instead of an object. This function is the first argument for the previous state, and the props are the second argument when the update is applied: 

this.setState(function(state, props) { 
  return { 
    counter: state.counter + props.increment 
  }; 
});

Is state changeable?

A state change takes place on the basis of the user input, which triggers an occurrence. React (with status) components are often made on the basis of state data. The initial knowledge is held by the State.

Thus when the state changes, React will be notified and the DOM will be re-rendered immediately; not the whole DOM but only the modified portion. This is one of the reasons for the fast reaction.

And how do you notify React? You thought: with setState ( ). The setState() method triggers the mechanism for rendering the modified components. React is notified, knows which part(s) to alter, and does so quickly without restoring the entire DOM.

Is state created in the component?

Let’s see the constructor method:

constructor() {
  super();
  this.state = {
    count: 0,
  };
}

This is where the state gets the initial data. The initial data (as above) can be hard-coded, but it can also come from props.

However, it makes sense – you cannot adjust props but the data a component receives wants to do so. This is where the state enters.

Component types

  • Stateless component — Just props, no state.  Besides the render() function, there’s not much going on and all its logic is about the props that they get. This makes it easy to track them (and test for that matter).  
  • The stately component — state as well as props. These are also called state managers. They are responsible for communication between clients and their servers (XHR, Web sockets, etc.), data processing, and user events.

What happens when state changes?

React Components allow you to break the UI into separate, reusable components so that you can look into every single item on an isolated basis.

Components are conceptually like functions in JavaScript. They accept arbitrary inputs and return elements of react that describe what should be shown on the screen. If you have to allow the user to enter something or to alter the variables that the component is supported by, you would have to setState.

State allows React components in response to user behavior, network responses, and everything else to adjust their performance over time, without violating this rule. Class-defined components provide additional functionality. Local status is the only class component function available.

Can I use state in every component?

In the early days, only class components, and not functional components, were included.

That’s why stateless components are known for their functional components. However, state can now be used in both class and functional components following the implementation of React Hooks.

You can only use status in class components if your project does not use React Hooks.

The component State and Props share some common similarities

Props and states both provide details about the item, but they are used differently and must be kept separate.

Conclusion

State refers to the component’s local status which cannot be used and changed outside the component and can only be used and modified within the component. On the other hand, it provides reusable components by enabling components to obtain data in the form of props from the parent component. 

We may change the state of a component with setState. These notifications are also triggered by events. setState is called asynchronous and merged with every entity in the current state. We may also transfer a setState function to allow us to write status changes based on the current status values. 

Most of your components can be stateless when you create an app. Props transfer parent-to-child data. They are unchangeable and are thus unchanged.

Abhresh

Abhresh Sugandhi

Author

Abhresh is specialized as a corporate trainer, He has a decade of experience in technical training blended with virtual webinars and instructor-led session created courses, tutorials, and articles for organizations. He is also the founder of Nikasio.com, which offers multiple services in technical training, project consulting, content development, etc.

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Handling React Events - A Detailed Guide

Event handling essentially allows the user to interact with a webpage and do something specific when a certain event like a click or a hover happens. When the user interacts with the application, events are fired, for example, mouseover, key press, change event, and so on. The application must handle events and execute the code. In short, events are the actions to which javascript can respond.   The actions to which javascript can respond are called events. Handling events with react is  very similar to handling events in DOM elements. Below are some general events that you would see in and out when dealing with react based websites:  Clicking an element  Submitting a form Scrolling page Hovering an element  Loading a webpage Input field change User stroking a key Image loading Naming Events in React Handling events with react is very similar to handling events in DOM elements, although there are some syntactic differences.   React events are written in camelCase.   A function is passed as the event handler rather than string. The way to write events in html / DOM is below:        click me onclick is written in lower case in html as shown above and what action to take when this onclick event triggers is taken care of by handleClick.In React, events are named using camel case and you pass a function as event handler as shown below:  Like in a functional component, event is written like below:       click me   In class based component ,event is written like below        click me Defining Events:Events are normally used in combination with functions, and the function is not executed until the event occurs, and the combination of event, HTML element, and javascript function is called binding which means to map all three. Generic syntax is:      Example:  Create a button element and what happens when onClick event triggered is driven by the function which is func() shown below     click me Let’s see some of the event attributes:   onmouseover : The mouse is moved over an element onmouseup : The mouse button is released onmouseout : The mouse  is moved off an element onmousemove: The mouse is moved Onmousedown: mouse button is pressed  onload : A image is done loading onunload: Existing the page  onblur : Losing Focus  on element  onchange : Content of a field changes onclick: Clicking an object  ondblclick: double clicking an object  onfocus element getting a focus  Onkeydown: pushing a keyboard key Onkeyup: keyboard key is released Onkeypress: keyboard key is pressed  Onselect: text is selected These are some examples of events:                                         Events                               function testApp (){                        alert((“Hello Event”);                                                   test Clicked                  test double Clicked                     Synthetic Events When you specify an event in JSX, you are not directly dealing with regular DOM events, you are dealing with a react event type called a synthetic event.It's a simple wrapper for native event instances and every synthetic event created needs to be garbage-collected which can be resource intensive in terms of CPU. The synthetic event object has properties mentioned below:  Boolean isTrusted  DOMEvent nativeEvent number timeStamp   void preventDefault() number eventPhase Synthetic events provide an interface and reduce browser inconsistencies and the event contains required information for its propagation to work. Synthetic event is reused for performance reasons in the browser, A synthetic event is a cross-browser wrapper around the browser’s native event it has the same interface as the native event. Synthetic events are delegated to the document node. Therefore native events are triggered first and the events bubble up to document, after which the synthetic events are triggered. The synthetic event object will be reused and all the properties will be nullified after the event callback has been invoked and this is for performance reasons.The workflow of synthetic event in react is:    Element ---- > Event ---- > synthetic event  ---- > handler(e)                                |                                                      |                                |  _______  Component ________|  umber timeStamp The Basics of React Event Handling Let’s explore how to handle events in react and we will showcase the click event and how it holds good for other types of events. Let’s start with functional components by creating a  file as clickAppHandler.js.In this file let’s create a  functional component  as shown below                        Import React from ‘...react’                         function clickAppHandler() {                                function clickHandler() {                                        console.log(‘clicked’)                                         }                                  return (                                                                                  Click                                                                          )                         }                       export default clickAppHandler  When onClick event triggers clickHandler function is called as shown below and when you click the button console will print the string “clicked” After this you need the add a component in the app component. In our code above you can see on click we pass the function as event handler and you will notice that we haven't added parentheses as it becomes a function, and we do not want that and we want handler to be a function not a function call. When a new component is rendered its event handler functions are added to the mapping maintained by the react.When the event is triggered and it hits and DOM object ,react maps the event to the handler, if it matches it calls the handler. The event handling in react is declarative and the advantage of declarative way to handlers is that they are part of the User interface structure.  Let’s take a look at event handling in class components                       Import React, { Component } from ‘...react’                         class TestApp extends Component {                              clickHandler() {                                  console.log(“clicked”)                                }                                render(){                                      return(                                                                                     Click me                                                                                 )                                 }                            }                       export default TestApp You cannot return false to prevent default behaviour in React. You must call preventDefault explicitly.  In HTML it looks like below:    Click Output: It will print “Clicked”  And in React, like this:  function clickHandle(e) {       e.preventDefault();       console.log(“Handled”);   }  Click  Output : console will print “Handled”  There are some  event handlers triggered by an event in the bubbling phase which is the same as with the normal DOM API; simply attach a handler to an eventual parent of an element and any events triggered on that element will bubble to the parent as long as it's not stopped via stopPropagation along the way   Click me  Below are some of the event handlers triggered in the bubbling phase:  MouseEvents           onClick           onDrag          onDoubleClick Keyboard Events                    onKeyDown                    onKeyPress                    onKeyUp Focus Events                  onFocus   onBlur To capture an event handler for the capture phase, append capture to the event name. For example, instead of using onClick, use onClickCapture to handle the click event.  Capture event example:                  Click me    Additional ExamplesExample1                       Import React from ‘...react’                         function clickAppHandler() {                                function clickHandler() {                                        console.log(‘clicked’)                                         }                                  return (                                                                                  Click                                                                          )                         }                       export default clickAppHandler   Example2       This example is along with HTML in a single file                                                            Events                               function testApp (){                        alert((“Hello Event”);                                                   test Clicked                  test double Clicked                     Adding Events: Below example is how you add an event. Highlighted in bold                      Import React from ‘...react’                         function clickAppHandler() {                                function clickHandler() {                                        console.log(‘clicked’)                                         }                                  return (                                                                                  Click                                                                          )                         }                       export default clickAppHandler  Passing Arguments to Event HandlerThere are two ways arguments are passed to event handler  Arrow function                    this.handleClick(id,e)}>Click                onClick is the event                e is the event object                 id can be state or props or some data Bind method      Click  In this case event object is automatically passed In both methods e represents the react event and its passed after the ID as second argument,With an arrow function this event e is passed explicitly but with bind method its automatically passed.                                     Import React,{ Component } from “react”;                                         class TestApp extends Component {                                           state = {                                                       id: 2,                                                      Name: “TestApp Dummy”                                                };                                                             //arrow function                                                 handleClick = (id,e) => {                                                       console.log(id);                                                       console.log(e);                                                  };                                               handleArg = (e) => { this.handleClick(this.state.id,e);}                                                          render() {     return (                    TestApp,{this.state.name}            onClick={this.handleArg}>Display            );   }  }  The react event is an object and obtained from react. Instead of creating a separate function for passing argument, you can directly pass the anonymous arrow function as shown in the render function below:     render() {        return (                                                                                                       TestApp,{this.state.name}                                                {                           this.handleClick(this.state.id,e);                                                               }}>Display                                                                                                         );                                                 }                                            }    Output:   click on button  “TestApp Dummy “                   Let’s see only how bind method looks like in the render function    render() {                                         return (                                                                                                 TestApp,{this.state.name}                                                   Display                                                                                                       );                                                  }                                              } Output: this will display the h1 tag and when you click the button handleClick function gets invoked and the console will display id of the state object as shown above. Building a Practice to Thoroughly Understand Events This blog focuses on event handling, which in turn teaches about event handlers declared in JSX markup.This approach helps in tracking down the element mapped with events in an easy way.  We also learned how to handle multiple event handlers in a single element by using JSX attributes.we also learned about ways to bind event handler and  parameter values. Then we learned about synthetic events which are abstractions around native events. The best way you can retain this learning is by practicing more and tackling the complexities that may arise as you practice. You can find several tutorials on the internet or share your questions with us here. Happy learning! 
5355
Handling React Events - A Detailed Guide

Event handling essentially allows the user to inte... Read More

MongoDB Query Document Using Find() With Example

MongoDB's find() method selects documents from a collection or view and returns a cursor to those documents. There are two parameters in this formula: query and projection.Query – This is an optional parameter that specifies the criteria for selection. In simple terms, a query is what you want to search for within a collection.Projection – This is an optional parameter that specifies what should be returned if the query criteria are satisfied. In simple terms, it is a type of decision-making that is based on a set of criteria.MongoDB's Flexible SchemaA NoSQL database, which stands for "not only SQL," is a way of storing and retrieving data that is different from relational databases' traditional table structures (RDBMS).When storing large amounts of unstructured data with changing schemas, NoSQL databases are indeed a better option than RDBMS. Horizontal scaling properties of NoSQL databases allow them to store and process large amounts of data.These are intended for storing, retrieving, and managing document-oriented data, which is frequently stored in JSON format (JavaScript Object Notation). Document databases, unlike RDBMSs, have a flexible schema that is defined by the contents of the documents.MongoDB is one of the most widely used open-source NoSQL document databases. MongoDB is known as a 'schemaless' database because it does not impose a specific structure on documents in a collection.MongoDB is compatible with a number of popular programming languages. It also offers a high level of operational flexibility because it scales well horizontally, allowing data to be spread or 'sharded' across multiple commodity servers with the ability to add more servers as needed. MongoDB can be run on a variety of platforms, including developer laptops, private clouds, and public clouds.Querying documents using find()MongoDB queries are used to retrieve or fetch data from a MongoDB database. When running a query, you can use criteria or conditions to retrieve specific data from the database.The function db.collection is provided by MongoDB. find() is a function that retrieves documents from a MongoDB database.In MongoDB, the find method is used to retrieve a specific document from the MongoDB collection. In Mongo DB, there are a total of six methods for retrieving specific records.find()findAndModify()findOne()findOneAndDelete()findOneAndReplace()findOneAndUpdate()Syntax:find(query, projection)We can fetch a specific record using the Find method, which has two parameters. If these two parameters are omitted, the find method will return all of the documents in the MongoDB collection.Example:Consider an example of employees with the database of employee_id and employee_name and we will fetch the documents using find() method.First, create a database with the name “employees” with the following code:use employeesNow, create a collection “employee” with:db.createCollection("employee")In the next step we will insert the documents in the database:db.employee.insert([{employee_id: 101, employee_name: "Ishan"}, {employee_id: 102, employee_name: "Bhavesh"}, {employee_id: 103, employee_name: "Madan"}])Find all Documents:To get all the records in a collection, we need to use the find method with an empty parameter. In other words, when we need all the records, we will not use any parameters.db.employee.find()Output in Mongo ShellThe pretty() method can be used to display the results in a formatted manner.Syntax:db.COLLECTION_NAME.find().pretty()Let’s check our documents with pretty() method:Query FiltersWe will see examples of query operations using the db.collection.find() method in mongosh.We will use the employee collection in the employees database.db.employee.insert([{employee_id: 101, employee_name: "Ishan", age: 21, email_id: "ishanjain@gmail.com"}, {employee_id: 102, employee_name: "Bhavesh", age: 22, email_id: "bhaveshg@gmail.com"}, {employee_id: 103, employee_name: "Madan", age: 23, email_id: "madan@gmail.com"}])As we have seen earlier that to select all the documents in the database we pass an empty document as the query filter parameter to the find method.db.employee.find().pretty()Find the first document in a collection:db.employee.findOne()Find a document by ID:db.employee.findOne({_id : ObjectId("61d1ae0b56b92c20b423a5a7")})Find Documents that Match Query Criteriadb.employee.find({“age”: “22”})db.employee.find({"employee_name": "Madan"}).pretty()Sort Results by a Field:db.employee.find().sort({age: 1}).pretty()order by age, in ascending orderdb.employee.find().sort({age: -1}).pretty()order by age, in descending orderAND Conditions:A compound query can specify conditions for multiple fields in the documents in a collection. A logical AND conjunction connects the clauses of a compound query indirectly, allowing the query to select all documents in the collection that meet the specified conditions.In the following example, we will consider all the documents in the employee collection where employee_id equals 101 and age equals 21.db.employee.find({"employee_id": 101, "age": "21" }).pretty()Querying nested fieldsThe embedded or nested document feature in MongoDB is a useful feature. Embedded documents, also known as nested documents, are documents that contain other documents.You can simply embed a document inside another document in MongoDB. Documents are defined in the mongo shell using curly braces (), and field-value pairs are contained within these curly braces.Using curly braces, we can now embed or set another document inside these fields, which can include field-value pairs or another sub-document.Syntax:{ field: { field1: value1, field2: value2 } }Example:We have a database “nested” and in this database we have collection “nesteddoc”.The following documents will insert into the nesteddoc collection.db.nesteddoc.insertMany([ { "_id" : 1, "dept" : "A", "item" : { "sku" : "101", "color" : "red" }, "sizes" : [ "S", "M" ] }, { "_id" : 2, "dept" : "A", "item" : { "sku" : "102", "color" : "blue" }, "sizes" : [ "M", "L" ] }, { "_id" : 3, "dept" : "B", "item" : { "sku" : "103", "color" : "blue" }, "sizes" : "S" }, { "_id" : 4, "dept" : "A", "item" : { "sku" : "104", "color" : "black" }, "sizes" : [ "S" ] } ])Place the documents in the collection now. Also, take a look at the results:As a result, the nesteddoc collection contains four documents, each of which contains nested documents. The find() method can be used to access the collection's documents.db.nesteddoc.find()Specify Equality Condition:In this example, we will select the document from the nesteddoc query where dept equals “A”.db.nesteddoc.find({dept: "A"})Querying ArraysUse the query document {: } to specify an equality condition on an array, where is the exact array to match, including the order of the elements.The following query looks for all documents where the field tags value is an array with exactly two elements, "S" and "M," in the order specified:db.nesteddoc.find( { sizes: ["S", "M"] } )Use the $all operator to find an array that contains both the elements "S" and "M," regardless of order or other elements in the array:db.nested.find( { sizes: { $all: ["S", "M"] } } )Query an Array for an Element:The following example queries for all documents where size is an array that contains the string “S” as one of its elements:db.nesteddoc.find( { sizes: "S" } )Filter conditionsTo discuss the filter conditions, we will consider a situation that elaborates this. We will start by creating a collection with the name “products” and then add the documents to it.db.products.insertMany([ { _id: 1, item: { name: "ab", code: "123" }, qty: 15, tags: [ "A", "B", "C" ] }, { _id: 2, item: { name: "cd", code: "123" }, qty: 20, tags: [ "B" ] }, { _id: 3, item: { name: "ij", code: "456" }, qty: 25, tags: [ "A", "B" ] }, { _id: 4, item: { name: "xy", code: "456" }, qty: 30, tags: [ "B", "A" ] }, { _id: 5, item: { name: "mn", code: "000" }, qty: 20, tags: [ [ "A", "B" ], "C" ] }])To check the documents, use db.products.find():$gt$gt selects documents with a field value greater than (or equal to) the specified value.db.products.find( { qty: { $gt: “20” } } )$gte:$gte finds documents in which a field's value is greater than or equal to (i.e. >=) a specified value (e.g. value.)db.products.find( { qty: { $gte: 20 } } )$lt:$lt selects documents whose field value is less than (or equal to) the specified value.db.products.find( { qty: { $lt: 25 } } )$lte:$lte selects documents in which the field's value is less than or equal to (i.e. =) the specified value.db.products.find( { qty: { $lte: 20 } } )Query an Array by Array Length:To find arrays with a specific number of elements, use the $size operator. For example, the following selects documents with two elements in the array.db.products.find( { "tags": {$size: 2} } )ProjectionIn MongoDB, projection refers to selecting only the data that is required rather than the entire document's data. If a document has five fields and you only want to show three of them, select only three of them.The find() method in MongoDB accepts a second optional parameter, which is a list of fields to retrieve, as explained in MongoDB Query Document. When you use the find() method in MongoDB, it displays all of a document's fields. To prevent this, create a list of fields with the values 1 or 0. The value 1 indicates that the field should be visible, while 0 indicates that it should be hidden.Syntax:db.COLLECTION_NAME.find({},{KEY:1})Example:We will consider the previous example of products collection. Run the below command on mongoshell to learn how projection works:db.products.find({},{"tags":1, _id:0})Keep in mind that the _id field is always displayed while executing the find() method; if you do not want this field to be displayed, set it to 0.Optimized FindingsTo retrieve a document from a MongoDB collection, use the Find method.Using the Find method, we can retrieve specific documents as well as the fields that we require. Other find methods can also be used to retrieve specific documents based on our needs.By inserting array elements into the query, we can retrieve specific elements or documents. To retrieve data for array elements from the collection in MongoDB, we can use multiple query operators.
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MongoDB Query Document Using Find() With Example

MongoDB's find() method selects documents from a c... Read More

Implementing MongoDb Map Reduce using Aggregation

Algorithms and applications in today's data-driven market collect data about people, processes, systems, and organisations 24 hours a day, seven days a week, resulting in massive amounts of data. The problem is figuring out how to process this massive amount of data efficiently without sacrificing valuable insights.What is Map Reduce? The MapReduce programming model comes to the rescue here. MapReduce, which was first used by Google to analyse its search results, has grown in popularity due to its ability to split and process terabytes of data in parallel, generating results faster. A (Key,value) pair is the basic unit of information in MapReduce. Before feeding the data to the MapReduce model, all types of structured and unstructured data must be translated to this basic unit. The MapReduce model, as the name implies, consists of two distinct routines: the Map-function and the Reduce-function.  MapReduce is a framework for handling parallelizable problems across huge files using a huge number of devices (nodes), which are collectively referred to as a cluster (if all nodes are on the same local network and use similar hardware) or a grid (if the nodes are shared across geographically and administratively distributed systems, and use more heterogeneous hardware).  When data stored in a filesystem (unstructured) or a database(structured) is processed, MapReduce can take advantage of data's locality, processing it close to where it's stored to reduce communication costs. Typically, a MapReduce framework (or system) consists of three operations: Map: Each worker node applies the map function to local data and saves the result to a temporary storage. Only one copy of the redundant input data is processed by a master node. Shuffle: worker nodes redistribute data based on output keys (produced by the map function), ensuring that all data associated with a single key is stored on the same worker node. Reduce: each group of output data is now processed in parallel by worker nodes, per key. This article will walk you through the Map-Reduce model's functionality step by step. Map Reduce in MongoDB The map-reduce operation has been deprecated since MongoDB 5.0. An aggregation pipeline outperforms a map-reduce operation in terms of performance and usability. Aggregation pipeline operators like $group, $merge, and others can be used to rewrite map-reduce operations. Starting with version 4.4, MongoDB provides the $accumulator and $function aggregation operators for map-reduce operations that require custom functionality. In JavaScript, use these operators to create custom aggregation expressions. The map and reduce functions are the two main functions here. As a result, the data is independently mapped and reduced in different spaces before being combined in the function and saved to the specified new collection. This mapReduce() function was designed to work with large data sets only. You can perform aggregation operations like max and avg on data using Map Reduce, which is similar to groupBy in SQL. It works independently and in parallel on data. Implementing Map Reduce with Mongosh (MongoDB Shell)  The db.collection.mapReduce() method in mongosh is a wrapper for the mapReduce command. The examples that follow make use of the db.collection.mapReduce(). Example: Create a collection ‘orders’ with these documents: db.orders.insertMany([     { _id: 1, cust_id: "Ishan Jain", ord_date: new Date("2021-11-01"), price: 25, items: [ { sku: "oranges", qty: 5, price: 2.5 }, { sku: "apples", qty: 5, price: 2.5 } ], status: "A" },     { _id: 2, cust_id: "Ishan Jain", ord_date: new Date("2021-11-08"), price: 70, items: [ { sku: "oranges", qty: 8, price: 2.5 }, { sku: "chocolates", qty: 5, price: 10 } ], status: "A" },     { _id: 3, cust_id: "Bhavesh Galav", ord_date: new Date("2021-11-08"), price: 50, items: [ { sku: "oranges", qty: 10, price: 2.5 }, { sku: "pears", qty: 10, price: 2.5 } ], status: "A" },     { _id: 4, cust_id: "Bhavesh Galav", ord_date: new Date("2021-11-18"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },     { _id: 5, cust_id: "Bhavesh Galav", ord_date: new Date("2021-11-19"), price: 50, items: [ { sku: "chocolates", qty: 5, price: 10 } ], status: "A"},     { _id: 6, cust_id: "Madan Parmar", ord_date: new Date("2021-11-19"), price: 35, items: [ { sku: "carrots", qty: 10, price: 1.0 }, { sku: "apples", qty: 10, price: 2.5 } ], status: "A" },     { _id: 7, cust_id: "Madan Parmar", ord_date: new Date("2021-11-20"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },     { _id: 8, cust_id: "Abhresh", ord_date: new Date("2021-11-20"), price: 75, items: [ { sku: "chocolates", qty: 5, price: 10 }, { sku: "apples", qty: 10, price: 2.5 } ], status: "A" },     { _id: 9, cust_id: "Abhresh", ord_date: new Date("2021-11-20"), price: 55, items: [ { sku: "carrots", qty: 5, price: 1.0 }, { sku: "apples", qty: 10, price: 2.5 }, { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },     { _id: 10, cust_id: "Abhresh", ord_date: new Date("2021-11-23"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" }  ]) Apply a map-reduce operation to the orders collection to group them by cust_id, then add the prices for each cust_id: To process each input document, define the map function: this refers the document that the map-reduce operation is processing in the function. For each document, the function maps the price to the cust_id and outputs the cust_id and price. var mapFunction1 = function() {emit(this.cust_id, this.price);}; With the two arguments keyCustId and valuesPrices, define the corresponding reduce function: The elements of the valuesPrices array are the price values emitted by the map function, grouped by keyCustId. The valuesPrice array is reduced to the sum of its elements by this function. var reduceFunction1 = function(keyCustId, valuesPrices) {return Array.sum(valuesPrices);};Apply the mapFunction1 map function and the reduceFunction1 reduce function to all documents in the orders collection: db.orders.mapReduce(mapFunction1,reduceFunction1,{ out: "map_reduce_example" }) The results of this operation are saved in the map_reduce_example collection. If the map_reduce_example collection already exists, the operation will overwrite its contents with the map-reduce operation's results. Check the map_reduce_example collection to verify: db.map_reduce_example.find().sort( { _id: 1 } ) Aggregation Alternative:You can rewrite the map-reduce operation without defining custom functions by using the available aggregation pipeline operators: db.orders.aggregate([{$group: { _id:"$cust_id",value:{$sum: "$price" } } },{ $out: "agg_alternative_1" }]) Check the agg_alternative_1 collection to verify: db.agg_alternative_1.find().sort( { _id: 1 } )Implementing Map Reduce with Java Consider the collection car and insert the following documents in it. db.car.insert( [ {car_id:"c1",name:"Audi",color:"Black",cno:"H110",mfdcountry:"Germany",speed:72,price:11.25}, {car_id:"c2",name:"Polo",color:"White",cno:"H111",mfdcountry:"Japan",speed:65,price:8.5}, {car_id:"c3",name:"Alto",color:"Silver",cno:"H112",mfdcountry:"India",speed:53,price:4.5}, {car_id:"c4",name:"Santro",color:"Grey",cno:"H113",mfdcountry:"Sweden",speed:89,price:3.5} , {car_id:"c5",name:"Zen",color:"Blue",cno:"H114",mfdcountry:"Denmark",speed:94,price:6.5} ] ) You will get an output like this:  Let's now write the map reduce function on a collection of cars, grouping them by speed and classifying them as overspeed cars.  var speedmap = function (){  var criteria;  if ( this.speed > 70 ) {criteria = 'overspeed';emit(criteria,this.speed);}}; Based on the speed, this function classifies the vehicle as an overspeed vehicle. The term "this" refers to the current document that requires map reduction. var avgspeed_reducemap = function(key, speed) {       var total =0;       for (var i = 0; i 
7344
Implementing MongoDb Map Reduce using Aggregation

Algorithms and applications in today's data-driven... Read More