What Is Objectid in Mongodb and How to Generate It Manually
3K- by KnowledgeHut
- 15th Feb, 2021
- Last updated on 11th Mar, 2021
- 7 mins read
MongoDB is a document-oriented NoSQL database used for high volume data storage. Instead of using tables and rows as in the traditional relational databases, MongoDB makes use of collections and documents.
You can read more about MongoDB here.
What are documents and collections?
- Collection: A collection is a group of MongoDB documents, and is similar to a table in a SQL(RDBMS) Database. A collection exists within a single database. Collections do not enforce a schema(structure). Documents within a collection can have different fields. Typically, all documents in a collection are of a similar or related purpose.
- Documents: A document is a set of key-value pairs. Documents have dynamic schema(structure). Dynamic schema means that documents in the same collection do not need to have the same set of fields or structure, and common fields in a collection's documents may hold different types of data.
An example of a MongoDB document is shown below:
{
_id: ObjectId(7df78ad8902ce46d)
title: 'Awesome Post',
description: 'This is an awesome post',
tags: ['tours', 'photography'],
likes: 100,
comments: [
{
user:'user1',
message: 'My first comment',
dateCreated: new Date(2011,1,20,2,15),
like: 0
},
{
user:'user2',
message: 'My second comments',
dateCreated: new Date(2011,1,25,7,45),
like: 5
}
]
}_id is a 12 bytes hexadecimal number which assures the uniqueness of every document. It is called ObjectId.
In this blog we are going to learn about MongoDB, ObjectId and how to generate it manually.
Why use MongoDB?
- It is document oriented: The document data model is a powerful way to store and retrieve data that allows developers to move fast.
- High performance: MongoDB’s horizontal, scale-out architecture can support huge volumes of both data and traffic.
Data Representation in JSON or BSON. An example of a JSON object is given below.
{
"name" : "Carlos Smith",
"title" : "Product Manager",
"location" : "New York, NY",
"twitter" : "@MongoDB",
"facebook" : "@MongoDB"
}You can read more about the use cases of MongoDB here.
What is the difference between MySQL and MongoDB?
The major and the key differences between MySQL and MongoDB are listed here:
MongoDB is an open-source database developed by MongoDB, Inc. MongoDB stores data in JSON-like documents that can vary in structure. It is a popular NoSQL database.
MySQL is a popular open-source relational database management system (RDBMS) that is developed, distributed and supported by Oracle Corporation. MySQL uses tables to store the data.You can read more about the differences here.
How to install MongoDB locally?
Let's go through the installation separately for Windows and Linux operating systems.
- Windows: Navigate to the below mentioned link and click on the download button to download the zip file, as you can see in the image.
Link to download MongoDB.
You can unzip and run the file to install MongoDB for Windows.
- Linux: You can simply run the below command inside the terminal to install MongoDB.
- Command: sudo apt-get install mongodb
Now, run the below command inside the command prompt, powershell or terminal to start MongoDB shell where we can create the Object IDs for this blog.
- Command: mongo
It's important to know that MongoDB uses BSON format to store data.
Difference between JSON and BSON:
As we have already seen how JSON looks, now let's take a look at BSON and understand the differences between JSON and BSON.
BSON stands for binary JSON (a superset of JSON with some more data types, most importantly binary byte array).
You can read more about the differences here.
What is ObjectId in MongoDB?
- As you see, in the example of a MongoDB document the _id field is called Object ID. MongoDB uses ObjectIds as the default value of _id field of each document, which is generated during the creation of any document.
- Object ID is treated as the primary key within any MongoDB collection. It is a unique identifier for each document or record.
- Syntax: ObjectId(<hexadecimal>).
- An ObjectId is a 12-byte BSON type hexadecimal string having the structure as shown in the example below.
Example: ObjectId("6009c0eee65f6dce28fb3e50") - The first 4 bytes are a timestamp value, representing the ObjectId’s creation, measured in seconds since the Unix epoch.
- Next 5-bytes represent a random value.
Note: Object ID is a unique identifier for each record which is created by declaring ObjectId as a method, as you will now see. Do not get confused between Object ID and ObjectId.
Creating a new ObjectId:
To create a new Object ID manually within the MongoDB you can declare ObjectId as a method. In the below image you can observe that we are declaring a variable having ObjectId method as a value. It will return a unique hexadecimal which we can store in a variable named myObjectId.
- Command: myObjectId = ObjectId()
In this example, the value of myObjectId would be: ObjectId("6009cb85e65f6dce28fb3e51").
In the above image, you can observe that each time it is returning a unique hexadecimal value.
Specify a Hexadecimal String:
Sometimes you want to define your own unique hexadecimal value, and MongoDB allows you to perform this action. In the above example the hexadecimal value is generated.
In this scenario, we will define an object ID with a hexadecimal value as a parameter of the ObjectId method.
- Command: newObjectId = ObjectId("507f1f77bcf86cd799439011")
In this example, the value of newObjectId would be: ObjectId("507f1f77bcf86cd799439011").
Access the Hexadecimal String:
In the above examples we are unable to get the hexadecimal string, as it will return you the whole method having the unique hexadecimal value.
To extract the unique hexadecimal as a string from the Object ID, we need to use ‘str’ attribute which is available in ObjectId method.
- Command: ObjectId("507f191e810c19729de860ea").str
In this example, the value of the ObjectId("507f191e810c19729de860ea").str method returned the hexadecimal string which is inside ObjectId method.
Methods available upon ObjectId:
- Note: We already have newObjectId with us so we can test these methods using the same variable.
- Method: ObjectId.getTimestamp().
- Explanation: Returns the timestamp portion of the object as a Date.
- Command: newObjectId.getTimestamp()
This will return the creation time of this document in ISO date format.
- Method: ObjectId.toString()
- Explanation: Returns the string representation of the ObjectId(). This string value has the format of ObjectId(<hexadecimal>).
- Command: newObjectId.toString()
This will return the ObjectId(<hexadecimal>) of this document in string format.
- Method: ObjectId.valueOf()
- Explanation: Returns the value of the ObjectId() as a lowercase hexadecimal string. This value is the str attribute of the ObjectId() object.
- Command: newObjectId.valueOf()
Note that the returned values of newObjectId.valueOf() and newObjectId.str are the same.
What you have learnt
- What is MongoDB and why do we use it?
- Differences between MySQL and MongoDB Databases
- How to install mongodb locally in Windows and Linux Operating systems.
- Differences between JSON and BSON documents
- What is ObjectId and its importance
- Generating a new ObjectId manually
- Methods available over Object Id and how to use them.
MongoDB is a NoSQL DB that is widely used in the industry, and also easy to learn. Good luck!
KnowledgeHut
KnowledgeHut is an outcome-focused global ed-tech company. We help organizations and professionals unlock excellence through skills development. We offer training solutions under the people and process, data science, full-stack development, cybersecurity, future technologies and digital transformation verticals.
Website : https://www.knowledgehut.com
Trending blog posts
Suggested Blogs
Handling React Events - A Detailed Guide
By Ankur Pandita
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
- by Ankur Pandita
- 21 Jan 2022
- 8 mins read
Event handling essentially allows the user to inte... Read More
MongoDB Query Document Using Find() With Example
By Abhresh Sugandhi
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.
6495
- by Abhresh Sugandhi
- 20 Jan 2022
- 8 mins read
MongoDB's find() method selects documents from a c... Read More
Implementing MongoDb Map Reduce using Aggregation
By Abhresh Sugandhi
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
- by Abhresh Sugandhi
- 19 Jan 2022
- 9 mins read
Algorithms and applications in today's data-driven... Read More
Join the Discussion
Your email address will not be published. Required fields are marked *