How to Aggregate Match in an Array in MongoDB

1. MongoDB Aggregate Match in an Array
2. Use $match With $in to Find Matching Documents in an Array in MongoDB
3. Use $match With $eq to Find Matching Documents in an Array in MongoDB
4. Use $match With $all to Find Matching Documents in an Array in MongoDB

This article educates about using the MongoDB aggregate match in an array with the help of code examples. We will explore different examples using string type array and numeric type array.

MongoDB Aggregate Match in an Array

We have already learned to use the match aggregation stage with comparison operators, $and/$or operators and $group/$project stages.

Here, we focus on learning the MongoDB aggregate match in an array to find matching documents. To create a collection where at least one field contains an array.

You can take advantage of the commands below to follow along with us.

Example Code:

db.createCollection('student_courses');
db.student_courses.insertMany([
    {"_id": "ma01", courses: ['Java', 'Python', 'Databases'], results: [3,5]},
    {"_id": "sd02", courses: ['Java', 'Python'], results: [6,3]},
    {"_id": "gh03", courses: ['JavaScript', 'Angular'], results: [8,9]},
    {"_id": "rt04", courses: ['Data Science', 'Python'], results: [2,5]}
]);
db.student_courses.find();

Output:

{ "_id" : "ma01", "courses" : [ "Java", "Python", "Databases" ], "results" : [ 3, 5 ] }
{ "_id" : "sd02", "courses" : [ "Java", "Python" ], "results" : [ 6, 3 ] }
{ "_id" : "gh03", "courses" : [ "JavaScript", "Angular" ], "results" : [ 8, 9 ] }
{ "_id" : "rt04", "courses" : [ "Data Science", "Python" ], "results" : [ 2, 5 ] }

Use $match With $in to Find Matching Documents in an Array in MongoDB

Example Code:

db.student_courses.aggregate({
    $match:{
        $expr:{ $in: ['Java', "$courses"]}
    }
});

Output:

{ "_id" : "ma01", "courses" : [ "Java", "Python", "Databases" ], "results" : [ 3, 5 ] }
{ "_id" : "sd02", "courses" : [ "Java", "Python" ], "results" : [ 6, 3 ] }

This code snippet uses the $match with $expr to let us use aggregation expressions within a query language. Here, the $expr builds the query expression that compares the fields from the exact documents in the $match stage.

Further, the $in operator selects those documents where the courses array contains Java as an array element. To check multiple values in the specified array, use the query in the following manner.

Example Code:

db.student_courses.aggregate({
    $match:{
        courses:{ $in: ['Java', 'Python']}
    }
});

Output:

{ "_id" : "ma01", "courses" : [ "Java", "Python", "Databases" ], "results" : [ 3, 5 ] }
{ "_id" : "sd02", "courses" : [ "Java", "Python" ], "results" : [ 6, 3 ] }
{ "_id" : "rt04", "courses" : [ "Data Science", "Python" ], "results" : [ 2, 5 ] }

This example selects only those documents where the courses array has either Java or Python (or both). Similarly, we can work with a numeric array.

Example Code:

db.student_courses.aggregate({
    $match:{
        results:{ $in: [3,6]}
    }
});

Output:

{ "_id" : "ma01", "courses" : [ "Java", "Python", "Databases" ], "results" : [ 3, 5 ] }
{ "_id" : "sd02", "courses" : [ "Java", "Python" ], "results" : [ 6, 3 ] }

This query retrieves only those documents where the results array contains either 3 or 6 (or both).

Example Code:

db.student_courses.aggregate({
    $match:{
       $expr:{ $gt: [{$sum: "$results"}, 10]}
    }
});

Output:

{ "_id" : "gh03", "courses" : [ "JavaScript", "Angular" ], "results" : [ 8, 9 ] }

Here, we select only those documents where the sum of all the elements of the results array is greater than 10.

Use $match With $eq to Find Matching Documents in an Array in MongoDB

Example Code:

db.student_courses.aggregate({
    $match:{
        courses:{ $eq: ['Java', 'Python']}
    }
});

Output:

{ "_id" : "sd02", "courses" : [ "Java", "Python" ], "results" : [ 6, 3 ] }

Here, we use the $eq operator to get only those elements where the courses array contains the exact array as we specify with the $eq operator.

Use $match With $all to Find Matching Documents in an Array in MongoDB

Example Code:

db.student_courses.aggregate({
    $match:{
        courses:{ $all: ['Java', 'Python']}
    }
});

Output:

{ "_id" : "ma01", "courses" : [ "Java", "Python", "Databases" ], "results" : [ 3, 5 ] }
{ "_id" : "sd02", "courses" : [ "Java", "Python" ], "results" : [ 6, 3 ] }

The $all is equivalent to the $and operator. This code retrieves all those documents where the courses array contains all the specified elements for the $all operator.

The resulting documents must contain Java and Python elements.