MongoDB query to get only specific fields in nested array documents?

To get only specific fields in nested array documents, use $filter along with $project. Let us create a collection with documents −

> db.demo342.insertOne({ ...    "Id": "101", ...       "details1" : { ...          "details2" : [ ...             { ...                "details3" : [ ...                   { ...                      "Name": "Mike", ...                      "CountryName" : "US" ...                   }, ...                   { ...                      "Name": "David", ...                      "CountryName" : "AUS" ...                   }, ...                   { ...                      "Name": "Bob", ...                      "CountryName" : "UK" ...                   } ...                ] ...             } ...          ] ...       } ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5e53ef99f8647eb59e5620a9") }

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

> db.demo342.find();

This will produce the following output −

{ "_id" : ObjectId("5e53ef99f8647eb59e5620a9"), "Id" : "101", "details1" : { "details2" : [ { "details3" : [ { "Name" : "Mike", "CountryName" : "US" }, { "Name" : "David", "CountryName" : "AUS" }, { "Name" : "Bob", "CountryName" : "UK" } ] } ] } }

Following is the query to get only specific fields in nested array documents −

> db.demo342.aggregate([ ...    { "$project": { ...       "details1": { ...          "details2": { ...             "$filter": { ...                "input": { ...                   "$map": { ...                      "input": "$details1.details2", ...                      "in": { ...                         "details3": { ...                            "$filter": { ...                               "input": "$$this.details3", ...                               "cond": { "$eq": ["$$this.Name", "Bob"] } ...                            } ...                         } ...                      } ...                   } ...                }, ...                "cond": { "$ne": ["$$this.details3", []] } ...             } ...          } ...       } ...    }} ... ])

This will produce the following output −

{ "_id" : ObjectId("5e53ef99f8647eb59e5620a9"), "details1" : { "details2" : [ { "details3" : [ { "Name" : "Bob", "CountryName" : "UK" } ] } ] } }