Let us first create a collection with documents −
>db.missingDocumentDemo.insertOne({"StudentFirstName":"Adam","StudentLastName":"Smith"}); { "acknowledged" : true, "insertedId" : ObjectId("5cd3fb1eedc6604c74817ce6") } >db.missingDocumentDemo.insertOne({"StudentFirstName":"Carol","StudentLastName":"Taylor"}); { "acknowledged" : true, "insertedId" : ObjectId("5cd3fb29edc6604c74817ce7") } >db.missingDocumentDemo.insertOne({"StudentFirstName":"David","StudentLastName":"Miller","StudentAge":21}); { "acknowledged" : true, "insertedId" : ObjectId("5cd3fb40edc6604c74817ce8") }
Following is the query to display all documents from a collection with the help of find() method −
> db.missingDocumentDemo.find().pretty();
This will produce the following output −
{ "_id" : ObjectId("5cd3fb1eedc6604c74817ce6"), "StudentFirstName" : "Adam", "StudentLastName" : "Smith" } { "_id" : ObjectId("5cd3fb29edc6604c74817ce7"), "StudentFirstName" : "Carol", "StudentLastName" : "Taylor" } { "_id" : ObjectId("5cd3fb40edc6604c74817ce8"), "StudentFirstName" : "David", "StudentLastName" : "Miller", "StudentAge" : 21 }
Here is the query to insert MongoDB document field only when it's missing. We are trying to insert StudentAge field here. It won’t insert the field if it already exist −
> db.missingDocumentDemo.update( ... { "StudentAge": { "$exists": false } }, ... { "$set": { "StudentAge": 23 } }, ... { "multi": true } ... ); WriteResult({ "nMatched" : 2, "nUpserted" : 0, "nModified" : 2 })
Let us display all the documents from the above collection −
> db.missingDocumentDemo.find().pretty();
This will produce the following output −
{ "_id" : ObjectId("5cd3fb1eedc6604c74817ce6"), "StudentFirstName" : "Adam", "StudentLastName" : "Smith", "StudentAge" : 23 } { "_id" : ObjectId("5cd3fb29edc6604c74817ce7"), "StudentFirstName" : "Carol", "StudentLastName" : "Taylor", "StudentAge" : 23 } { "_id" : ObjectId("5cd3fb40edc6604c74817ce8"), "StudentFirstName" : "David", "StudentLastName" : "Miller", "StudentAge" : 21 }