MongoDB and Mongoose - Chain Search Query Helpers to Narrow Search Results
If you don’t pass the callback as the last argument to Model.find() (or to the other search methods), the query is not executed. You can store the query in a variable for later use. This kind of object enables you to build up a query using chaining syntax. The actual db search is executed when you finally chain the method .exec(). Pass your callback to this last method. There are many query helpers, here we’ll use the most ‘famous’ ones.
Find people who like "burrito". Sort them by name, limit the results to two documents, and hide their age. Chain .find(), .sort(), .limit(), .select(), and then .exec(). Pass the done(err, data) callback to exec().