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An Introduction to Angular Components

Every Application has an important building block, the UI, and a Component in Angular is the basic unit which helps to build a UI. In general user terms UI is referred to as a view. What are Components? Components are basic building blocks which consist of an HTML template that declares what renders on the page, a class written in TypeScript which defines the behaviour, a CSS selector which defines how it can be used in the template and CSS styles which define how it will look when a user views it. Representation of Components:If you’re a programmer by profession and have some fundamental knowledge of user experience, the above diagram will help you to understand about the internal structures of components. Let’s take an example to understand components in a better way, and consider that we are building a page for an application. The features in the page include the header, footer and navigation and content area. Instead of building a single page with all these features, we can choose to split the page into components, which help us to manage our application.In the above scenario, we can say that the header, footer, content area, navigation and so on are separate components of the page; but when the user views it on the website through any device, it will show as a single page.Next, we will find out how to build components, and what is the internal structure of components.The Structure of Components: Components are comprised of Template, Directives and Data bindings. Before creating a component, we should get an idea of what these are. Template - A template combines HTML with Angular mark-up, to modify HTML elements before they are displayed. It’s an HTML file which displays information. Here is an example of a Template: Directives – Directives form an important part of a component. They provide the program logic to the component. Here are some examples of directives: *ngFor, *ngIf Data Binding – Data binding is the process that establishes a connection between the application UI and business logic. Without this, no component in Angular can become functional. Here is an example of Data Binding: ‹div> <span>Current Number is {{value1}}</span> <br><br> <span>Current Number is {{value1 | currency}}</span> <br /><br /> <span>Current Number is {{dt1}}</span> <br /><br /> <span>Current Number is {{dt1 | date}}</span> <br /><br /> <span>Status is {{status}}</span> <br /><br /> </div>Component metadata The metadata for a component tells Angular where to get the major building blocks that it needs to create and present the component and its view. In particular, it associates a template with the component, either directly with inline code, or by reference. Together, the component and its template describe a view. Example of metadata:@Component({  selector:  app-hero-list',  templateUrl:'./hero-list. component. html',  providers:  HeroService  }) Here in the above example, the selector, template URL, and providers consist of metadata which tell where to get the major building blocks.  Next, let us learn how we can configure a component in a project. Configuration of Angular Component The easiest way to create a component is with the Angular CLI. You can also create a component manually.  Creating a component using the Angular CLIFrom a terminal window, navigate to the directory containing your application. Run the ng generate component <component-name> command, where <component-name> is the name of your new component. By default, this command creates the following: A folder named after the component A component file, <component-name>.component.ts A template file, <component-name>.component.html A CSS file, <component-name>.component.css A testing specification file, <component-name>.component.spec.ts Where <component-name> is the name of your component. Creating a component manually Navigate to your Angular project directory. Create a new file, <component-name>.component.ts. At the top of the file, add the following import statement. import { Component } from '@angular/core'; After the import statement, add a @Component decorator. @Component({  }) Choose a CSS selector for the component. @Component({   selector: 'app-component-overview',  }) Define the HTML template that the component uses to display information. In most cases, this template is a separate HTML file. @Component({  selector: 'app-component-overview',   templateUrl: './component-overview.component.html',  }) Select the styles for the component's template. In most cases, you define the styles for your component's template in a separate file. @Component({   selector: 'app-component-overview',  templateUrl: './component-overview.component.html',  styleUrls: ['./component-overview.component.css']  }) Add a class statement that includes the code for the component. export class Component Overview Component {  } For more details information on creating an Angular component, the reader can go through the official website of Angular refWe are now going to learn another concept in terms of the uses of Angular components. Let’s talk about the use of Aliases for importing components. Aliases in Angular component imports  When working with Angular projects in real time, we might come across a scenario where we are required to remember a long relative path for a component in an Angular file. This often proves to be difficult, and it makes the application messy and complex to read, especially if you are new to the team. This is where Aliases come to the rescue. You might come across something like:In the above example, we should use aliases for relative paths to improve the readability of the code. To achieve this in your Angular application, all you need to do is to update the tsconfig.json file. If you look at the above configuration, the default property of the baseUrl property was updated to ‘src’ directory. Then, we added a new property called paths, which is an object containing key-value pairs representing aliases defined for the paths in our application. The above code can be rewritten as shown below: Like Aliases help in code readability, Lazy load is also a very useful Angular feature. Let’s Understand Lazy Load in terms of any programming language: As the term itself suggests, a Lazy Load is loaded late and only when needed. To understand this in a better way, consider a VIEW MORE button on any web page of an application. When we click on this VIEW MORE button, it loads the rest of the content and displays it to the user. In a similar way, in Angular applications, we have several such components which are not very important. Only when the user wants to view these components, they must be loaded. We use Lazy load in such scenarios, as using this feature will make the best use of time and space in applications. There are two main steps to setting up a lazy-loaded feature module: Create the feature module with the CLI, using the --route flag. Configure the routes. To understand in more detail, with some sample code, please go through this reference URL where it is explained very well by the Angular TeamIn the example below, the LOAD MORE Button is created using the lazy loading concept. Visitors can view more blogs only if they are interested by clicking on this button.Conclusion: I hope this blog will help readers to understand some basic concepts of Angular components and their structure. To explore more advanced concepts and delve deeper into Angular 2-11,  visit this link.

An Introduction to Angular Components

9K
  • by Jaya Jha
  • 20th Apr, 2021
  • Last updated on 21st Apr, 2021
  • 6 mins read
An Introduction to Angular Components

Every Application has an important building block, the UI, and Component in Angular is the basic unit which helps to build a UI. In general user terms UI is referred to as view. 

What are Components? 

Components are basic building blocks which consist of aHTML template that declares what renders on the page, a class written in TypeScript which defines the behaviour, CSS selector which defines how it can be used in the template and CSS styles which define how it will look when a user views it. 

Representation of Components:

Representation of Components

If you’re a programmer by profession and have some fundamental knowledge of user experience, the above diagram will help you to understand about the internal structures of components. 

Let’s take an example to understand components in a better way, and consider that we are building a page for an application. The features in the page include the header, footer and navigation and content area. Instead of building a single page with all these features, we can choose to split the page into components, which help us to manage our application.

In the above scenario, we can say that the header, footer, content area, navigation and so on are separate components of the page; but when the user views it on the website through any device, it will show as a single page.

Next, we will find out how to build components, and what is the internal structure of components.

The Structure of Components: 

Components are comprised of Template, Directives and Data bindings. Before creating a component, we should get an idea of what these are. 

Template - A template combines HTML with Angular mark-upto modify HTML elements before they are displayed. 

It’s an HTML file which displays information. 

Here is an example of Template: example of a Template

Directives  Directives form an important part of component. They provide the program logic to the component. 

Here are some examples of directives: *ngFor, *ngIf 

Data Binding – Data binding is the process that establishes a connection between the application UI and business logic. Without this, no component in Angular can become functional. 

Here is an example of Data Binding: 

‹div> 
<span>Current Number is {{value1}}</span> 
<br><br> 
<span>Current Number is {{value1 | currency}}</span> 
<br /><br /> 
<span>Current Number is {{dt1}}</span> 
<br /><br /> 
<span>Current Number is {{dt1 | date}}</span> 
<br /><br /> 
<span>Status is {{status}}</span> 
<br /><br /> 
</div>

Component metadata 

The metadata for a component tells Angular where to get the major building blocks that it needs to create and present the component and its view. In particular, it associates a template with the component, either directly with inline code, or by reference. Together, the component and its template describe a view. 

Example of metadata:

@Component({ 
selector: 
app-hero-list', 
templateUrl:'./hero-list. component. html', 
providers: 
HeroService 
}) 

Here in the above example, the selector, template URL, and providers consist of metadata which tell where to get the major building blocks.  

Next, let us learn how we can configure component in a project. 

Configuration of Angular Component 

The easiest way to create a component is with the Angular CLI. You can also create a component manually.  

Creating a component using the Angular CLI

From a terminal window, navigate to the directory containing your application. 

Run the ng generate component <component-name> command, where <component-name> is the name of your new component. 

By default, this command creates the following: 

  • A folder named after the component 
  • A component file, <component-name>.component.ts 
  • A template file, <component-name>.component.html 
  • A CSS file, <component-name>.component.css 
  • A testing specification file, <component-name>.component.spec.ts 
  • Where <component-name> is the name of your component. 

Creating a component manually 

  • Navigate to your Angular project directory. 
  • Create a new file, <component-name>.component.ts. 
  • At the top of the file, add the following import statement. 
import { Component } from '@angular/core'; 
  1. After the import statement, add a @Component decorator. 

@Component({ 
}) 
  1. Choose a CSS selector for the component. 

@Component({ 
 selector: 'app-component-overview', 
}) 
  1. Define the HTML template that the component uses to display information. In most cases, this template is a separate HTML file. 

@Component({ 
selector: 'app-component-overview', 
 templateUrl: './component-overview.component.html', 
}) 
  1. Select the styles for the component's template. In most cases, you define the styles for your component's template in a separate file. 

@Component({ 
 selector: 'app-component-overview', 
templateUrl: './component-overview.component.html', 
styleUrls: ['./component-overview.component.css'] 
}) 
  1. Add a class statement that includes the code for the component. 

export class Component Overview Component { 
} 

For more details information on creating an Angular component, the reader can go through the official website of Angular ref

Ware now going to learn another concept in terms of the uses of Angular components. Let’s talk about the use of Aliases for importing components. 

Aliases in Angular component imports  

When working with Angular projects in real time, we might come across a scenario where we are required to remember a long relative path for a component in an Angular file. This often proves to be difficult, and it makes the application messy and complex to read, especially if you are new to the team. This is where Aliases come to the rescue. 

You might come across something like:Aliases in Angular component imports

In the above example, we should use aliases for relative paths to improve the readability of the code. 

To achieve this in your Angular application, all you need to do is to update the tsconfig.json file. 

If you look at the above configuration, the default property of the baseUrl property was updated to ‘src’ directory. Then, we added a new property called paths, which is an object containing key-value pairs representing aliases defined for the paths in our application. 

The above code can be rewritten as shown below: An Introduction to Angular Components

Like Aliases help in code readability, Lazy load is also a very useful Angular feature. 

Let’s Understand Lazy Load in terms of any programming language: 

As the term itself suggests, Lazy Load is loaded late and only when needed. To understand this in a better way, consider a VIEW MORE button on any web page of an application. When we click on this VIEW MORE button, it loads the rest of the content and displays it to the user. 

In a similar way, in Angular applicationswe have several such components which are not very important. Only when the user wants to view these components, they must be loaded. We use Lazy load in such scenarios, as using this feature will make the best use of time and space in applications. 

There are two main steps to setting up a lazy-loaded feature module: 

  • Create the feature module with the CLI, using the --route flag. 
  • Configure the routes. 

To understand in more detailwith some sample code, please go through this reference URL where it is explained verwell by the Angular Team

In the example below, the LOAD MORE Button is created using the lazy loading concept. Visitors can view more blogs only if they are interested by clicking on this button.KH Blog page view

Conclusion: 

I hope this blog will help readers to understand some basic concepts of Angular components and their structure. To explore more advanced concepts and delve deeper into Angular 2-11,  visit this link.

Jaya

Jaya Jha

Author

I am a full-stack Web Application Developer and passionate about exploring cutting edge technologies. I love to write technical blogs.

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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