Find MongoDB documents that contains specific field?

To find documents that contain specific field, use $exists. Let us create a collection with documents −

> db.demo247.insertOne({"ClientDetails":[{"ClientFirstName":"Chris","ClientAge":34}]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e46b2a31627c0c63e7dba69") } >db.demo247.insertOne({"ClientDetails":[{"ClientFirstName":"John","ClientLastName":"Smith","ClientAge":31}]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e46b2be1627c0c63e7dba6a") } > db.demo247.insertOne({"ClientDetails":[{"ClientFirstName":"David","ClientAge":33}]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e46b2cd1627c0c63e7dba6b") } >db.demo247.insertOne({"ClientDetails":[{"ClientFirstName":"David","ClientLastName":"Miller","ClientAge":31}]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e46b2de1627c0c63e7dba6c") }

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

> db.demo247.find().pretty();

This will produce the following output −

{    "_id" : ObjectId("5e46b2a31627c0c63e7dba69"),    "ClientDetails" : [       {          "ClientFirstName" : "Chris",          "ClientAge" : 34       }    ] } {    "_id" : ObjectId("5e46b2be1627c0c63e7dba6a"),    "ClientDetails" : [       {          "ClientFirstName" : "John",          "ClientLastName" : "Smith",          "ClientAge" : 31       }    ] } {    "_id" : ObjectId("5e46b2cd1627c0c63e7dba6b"),    "ClientDetails" : [       {          "ClientFirstName" : "David",          "ClientAge" : 33       }    ] } {    "_id" : ObjectId("5e46b2de1627c0c63e7dba6c"),    "ClientDetails" : [       {          "ClientFirstName" : "David",          "ClientLastName" : "Miller",          "ClientAge" : 31       }    ] }

Following is the query to find documents that contains specific field −

> db.demo247.find({"ClientDetails.ClientLastName":{$exists:true}});

This will produce the following output −

{    "_id" : ObjectId("5e46b2be1627c0c63e7dba6a"), "ClientDetails" : [       { "ClientFirstName" : "John", "ClientLastName" : "Smith", "ClientAge" : 31 }    ] } {    "_id" : ObjectId("5e46b2de1627c0c63e7dba6c"), "ClientDetails" : [       { "ClientFirstName" : "David", "ClientLastName" : "Miller", "ClientAge" : 31 }    ] }