MongoDB - Projection Queries

A projection query is a query where you specify which fields should be returned.

In MongoDB, when you query a collection using the db.collection.find() method, you can specify which fields you would like to have returned.

You can do this by including the field names in your query, and adding a 1 or 0 next to them, to specify whether it should be returned or not. This is a projection parameter. A projection parameter of 1 will display the field and a 0 will hide it.


First let's do a query without projection (so we can see how many fields are returned):

Without Projection


{ "_id" : 1, "name" : "Ian Gillan", "instrument" : "Vocals" }
{ "_id" : 6, "name" : "Jeff Martin", "instrument" : "Vocals", "born" : 1969 }

With Projection

Now, let's use projection to display just the name field:


{ "_id" : 1, "name" : "Ian Gillan" }
{ "_id" : 6, "name" : "Jeff Martin" }

You will notice that the _id field is automatically included, even if you don't specify it. You can exclude this field by using a 0 against it:


{ "name" : "Ian Gillan" }
{ "name" : "Jeff Martin" }

Mixing Inclusions and Exclusions

You can't mix 1s and 0s (with the exception of the _id field). If you try to mix inclusions and exclusions, like this:

You'll end up with this error:

Error: error: {
	"waitedMS" : NumberLong(0),
	"ok" : 0,
	"errmsg" : "Projection cannot have a mix of inclusion and exclusion.",
	"code" : 2

So to you either include fields or exclude them — not both.

Here's an example of specifying fields by exclusion:


{ "name" : "Ian Gillan" }
{ "name" : "Jeff Martin", "born" : 1969 }