How to filter some fields in objects and fetch a specific subject name value in MongoDB?

To filter and fetch, use projection along with MongoDB $filter and $match. Let us create a collection with documents −

> db.demo507.insertOne( ... { ... ...    "Information": ...    [ ...       {"Name":"John","SubjectName":"MySQL"}, ...       {"Name":"Bob","SubjectName":"MongoDB"}, ...       {"Name":"Chris","SubjectName":"MySQL"}, ...       {"Name":"David","SubjectName":"C++"} ...    ] ... } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5e8836d3987b6e0e9d18f577") }

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

> db.demo507.find().pretty();

This will produce the following output −

{    "_id" : ObjectId("5e8836d3987b6e0e9d18f577"),    "Information" : [       {          "Name" : "John",          "SubjectName" : "MySQL"       },       {          "Name" : "Bob",          "SubjectName" : "MongoDB"       },       {          "Name" : "Chris",          "SubjectName" : "MySQL"       },       {          "Name" : "David",          "SubjectName" : "C++"       }    ] }

Following is the query to filter some fields in objects −

> db.demo507.aggregate([ ...    {$match: {"Information.SubjectName" : "MySQL" } }, ...    {$project: { ...       _id:0, ...       Information: { ...          $filter: { ...             input: '$Information', ...             as: 'result', ...             cond: {$eq: ['$$result.SubjectName', 'MySQL']} ...          } ...       } ...    } ... },{$project: {Information: { SubjectName:1}}} ... ]);

This will produce the following output −

{ "Information" : [ { "SubjectName" : "MySQL" }, { "SubjectName" : "MySQL" } ] }