When you use methods such as find() or findOne() in MongoDB, by default you get the whole document returned. And if you use projections, you can return specific key/value pairs.

But what if you only want the value?

You can extract the value of a field by appending that field’s name to your query when using findOne().

Example

Suppose we have a collection called products with the following documents:

{ "_id" : 1, "product" : "Bat", "sizes" : [ "S", "M", "L" ] }
{ "_id" : 2, "product" : "Hat", "sizes" : [ "S", "L", "XL" ] }
{ "_id" : 3, "product" : "Cap", "sizes" : [ "M", "L" ] }

If we wanted to return the first document’s product, we could do the following:

db.products.findOne().product

Result:

Bat

Note that we used the findOne() method. This technique doesn’t work on the find() method. The findOne() method returns a single document, whereas the find() method merely returns a cursor to the document.

If we wanted to return the array we could do this:

db.products.findOne().sizes

Result:

[ "S", "M", "L" ]

And we can get an array value by referencing its index:

db.products.findOne().sizes[0]

Result:

S

Arrays are zero-based, and so 0 references the first element in the array, 1 references the second element, 2 the third, and so on.

Specific Document

By default, the findOne() method returns the first document in the collection. We can select a different document by specifying passing a query as the first argument.

I say “first argument” because findOne() also accepts a projection argument as an optional second argument.

db.products.findOne({_id: 2}, {sizes: 1, _id: 0}).sizes

Result:

[ "S", "L", "XL" ]

In this case I added a projection argument, but it had no effect on the result. But it would have had an effect if I’d specified a value of 0. That would have resulted in nothing being returned.

Embedded Documents

You can use dot notation to return values from embedded documents

Example document:

{
	"_id" : 1,
	"name" : "Wag",
	"details" : {
		"type" : "Dog",
		"weight" : 20
	}
}

We could do the following to return a value from the embedded document:

db.pets.findOne().details.type

Result:

Dog