How can I find documents in MongoDB based on the number of matched objects within an array?

Let us see an example and create a collection with documents −

> db.demo694.insertOne( ...    { ...       "details" : ...       [ ...          { ...             "Name" : "Chris", ...             Age:21 ...          }, ...          { ...             "Name" : "David", ...             Age:22 ...          } ...       ] ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea59279ece4e5779399c07f") } > db.demo694.insertOne( ...    { ...       "details" : ...       [ ...          { ...             "Name" : "Bob", ...             Age:21 ...          }, ...          { ...             "Name" : "Mike", ...             Age:23 ...          } ...       ] ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea5927aece4e5779399c080") } > db.demo694.insertOne( ...    { ...       "details" : ...       [ ...          { ...             "Name" : "Chris", ...             Age:21 ...          }, ...          { ...             "Name" : "Carol", ...             Age:22 ...          } ...       ] ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5ea5927aece4e5779399c081") }

Display all documents from a collection with the help of find() method −

> db.demo694.find();

This will produce the following output −

{ "_id" : ObjectId("5ea59279ece4e5779399c07f"), "details" : [ { "Name" : "Chris", "Age" : 21 }, { "Name" : "David", "Age" : 22 } ] } { "_id" : ObjectId("5ea5927aece4e5779399c080"), "details" : [ { "Name" : "Bob", "Age" : 21 }, { "Name" : "Mike", "Age" : 23 } ] } { "_id" : ObjectId("5ea5927aece4e5779399c081"), "details" : [ { "Name" : "Chris", "Age" : 21 }, { "Name" : "Carol", "Age" : 22 } ] }

Following is the query to find documents in MongoDB based on the number of matched objects within an array −

> nameChrisAge21= function () { ...    var inc = 0; ...    this.details.forEach(function (d) { ...       if (d.Name == "Chris" && d.Age == 21) { ...          inc= inc + 1; ...       } ...    }); ...    if (inc >= 1) { ...       return true; ...    } ... } function () {    var inc = 0;    this.details.forEach(function (d) {       if (d.Name == "Chris" && d.Age == 21) {          inc= inc + 1;       }    });    if (inc >= 1) {       return true;    } } > db.demo694.find({$where:nameChrisAge21});

This will produce the following output −

{ "_id" : ObjectId("5ea59279ece4e5779399c07f"), "details" : [ { "Name" : "Chris", "Age" : 21 }, { "Name" : "David", "Age" : 22 } ] } { "_id" : ObjectId("5ea5927aece4e5779399c081"), "details" : [ { "Name" : "Chris", "Age" : 21 }, { "Name" : "Carol", "Age" : 22 } ] }