How to determine whether a field exists in MongoDB?

You need to use $exists operator to determine whether a field exists in MongoDB. Let us first create a collection with documents

> db.determineFieldExistsDemo.insertOne({"ClientName":"John"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9eb245d628fa4220163b75") } > db.determineFieldExistsDemo.insertOne({"ClientName":"Larry","ClientAge":23}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9eb25cd628fa4220163b76") } > db.determineFieldExistsDemo.insertOne({"ClientName":"Mike","ClientCountryName":"UK"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9eb26fd628fa4220163b77") } > db.determineFieldExistsDemo.insertOne({"ClientName":"Sam","ClientAge":24}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9eb286d628fa4220163b78") }

Following is the query to display all documents from a collection with the help of find() method

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

This will produce the following output

{ "_id" : ObjectId("5c9eb245d628fa4220163b75"), "ClientName" : "John" } {    "_id" : ObjectId("5c9eb25cd628fa4220163b76"),    "ClientName" : "Larry",    "ClientAge" : 23 } {    "_id" : ObjectId("5c9eb26fd628fa4220163b77"),    "ClientName" : "Mike",    "ClientCountryName" : "UK" } {    "_id" : ObjectId("5c9eb286d628fa4220163b78"),    "ClientName" : "Sam",    "ClientAge" : 24 }

Following is the query to determine whether a field exists

> db.determineFieldExistsDemo.find({ClientCountryName:{$exists:true}}).pretty();

This will produce the following output

{    "_id" : ObjectId("5c9eb26fd628fa4220163b77"),    "ClientName" : "Mike",    "ClientCountryName" : "UK" }