[mongodb] How to select a single field for all documents in a MongoDB collection?

I just want to add to the answers that if you want to display a field that is nested in another object, you can use the following syntax

db.collection.find( {}, {{'object.key': true}})

Here key is present inside the object named object

{ "_id" : ObjectId("5d2ef0702385"), "object" : { "key" : "value" } }