How to use projection operators in MongoDB to filter data
Document-oriented database, MongoDB is a type of NoSQL database. Different types of operators are available in MongoDB for usage when interacting with the database.
Projection operators perform the different operations on the array to project elements. The projection operators available in MongoDB are listed below:
$$elemMatch$meta$slice
We'll use the following database to perform different projection operators where the database name is educative, and it has a collection named courses.
use educative //selecting our databasedb.courses.find({}) //showing all the documents of "courses"[{ _id: 10, course_name: 'python', hours: [10,14,20] },{ _id: 11, course_name: 'C++', hours: [10,15] },{ _id: 12, course_name: 'java', hours: [10,11,12] }]
Note: To learn about array operators in MongoDB, click here.
The $operator
The $ operator is used to match the first element from an array that is specified in the given query condition. Only a single $ operator can be used in one query.
Syntax
The syntax of $ operator is:
db.collection.find({ <array.field>: <condition> ...},{ "<array>.$": 1 })
Code example
Here's an example of $ operator:
//querydb.courses.find( {hours: { $gte: 12}}, {"hours.$": 1})//output[{ _id: 10, hours: [ 14 ] },{ _id: 11, hours: [ 15 ] },{ _id: 12, hours: [ 12 ] }]
The above query returns only the first value of hours from each document that is greater or equal to 12.
The $elemMatchoperator
The $elemMatch operator matches documents with an array field with at least one element that satisfies all the specified query criteria.
Syntax
Here's the syntax of $elemMatch operator:
{<field>:{$elemMatch:{<query1>, <query2>, ...}}}
Code example
An example of the $elemMatch operator is given below:
//querydb.courses.find({ "hours": { $elemMatch: {$gte: 11, $lt: 15}}}).pretty()//output[{ _id: 10, course_name: 'python', hours: [ 10, 14, 20 ] },{ _id: 12, course_name: 'java', hours: [ 10, 11, 12 ] }]
The $elemMatch query returns only those hours values that are greater than or equal to 11 and less than 15.
The$metaoperator
The $meta operator is used to see the metadata or description of the document while searching text.
Syntax
The syntax of $meta operator is:
{ $meta: <metaDataKeyword> }
The $meta operator can be used with keyword textScore that returns the score of associated $text query for each matching document.
Note: We can use
$metawithin asort()function but thetextScoremetadata will be sorted in descending order.
Example
Here's an example of $meta operator:
Step 1: Create an index for the field course_name:
db.courses.createIndex({"course_name": "text"})
Step 2: Run the $meta query to search text:
//querydb.courses.find({$text : {$search: "python"}},{score: {$meta: "textScore"},_id: 0})//output[ { course_name: ‘python’, hours: [ 10, 14, 20 ], score: 1.1 } ]
The above query returns the document that contains the text python and textScore associated with this text.
Note: The
_id: 0field is used to omit theobject_idof each document for a more straightforward output.
The $sliceoperator
The $slice operator is used to limit or skip the number of elements in an array that are specified in the query.
Syntax
The syntax of $slice operator is:
db.collection.find(<query>,{ <arrayField>: { $slice: <number> } });//ordb.collection.find(<query>,{ <arrayField>: { $slice: [ <number to skip>, <number to return> ] } });
Example
Here's an example of $slice operator:
//querydb.courses.find( { }, { _id: 0, "hours": { $slice: 1 } } )//output[{ course_name: 'python', hours: [ 10 ] },{ course_name: 'C++', hours: [ 10 ] },{ course_name: 'java', hours: [ 10 ] }]
The above query returned only the first values of the hours as we have used 1 in the query.
Run your queries
You can run all the aforementioned MongoDB queries in the terminal below:
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