16 down vote accepted

Basically when using mongoose, documents can be retrieved using helpers. Every model method that accepts query conditions can be executed by means of acallbackor theexecmethod.

callback:

User.findOne({ name:'daniel'},function(err, user){//});

exec:

User.findOne({ name:'daniel'}).exec(function(err, user){//});

Therefore when you don't pass a callback you can build a query and eventually execute it.

You can find additional info in themongoose docs.

UPDATE

Something to note when usingPromisesin combination with Mongoose async operations is that Mongoose queries arenotPromises. Queries do return athenable, but if you need arealPromise you should use theexecmethod. More information can be foundhere.

16 down vote accepted

Basically when using mongoose, documents can be retrieved using helpers. Every model method that accepts query conditions can be executed by means of acallbackor theexecmethod. callback: User.findOne({ name:'daniel'},function(err, user){//}); exec: User.findOne({ name:'daniel'}).exec(function(err, user){//}); Therefore when you don't pass a callback you can build a query and eventually execute it. You can find additional info in themongoose docs. UPDATE Something to note when usingPromisesin combination with Mongoose async operations is that Mongoose queries arenotPromises. Queries do return athenable, but if you need arealPromise you should use theexecmethod. More information can be foundhere.

Daniel has answered this quite beautifully. To elaborate on an exhaustive list of ways to build and execute queries, look at the following use cases:

Query Building

Mongoose will not execute a query untilthenorexechas been called upon it. This is very useful when building complex queries. Some examples can include using thepopulateandaggregatefunctions.

User.find({name: 'John'}) // Will not execute

Execution via callback

Although disliked by many due to its nesting nature, queries can be executed by providing the optional callback.

User.find({name: 'John'}, (err, res) => {}) // Will execute

Then API as a Promises/A+

Mongoose queries do provide athenfunction. This is not to be confused with regular promises. Simply put, the Promises/A+ specification requires athenfunction to work much like how we're used to with promises.

User.find({name: 'John'}).then(); // Will execute
Promise.all([User.find({name: 'John'}), User.find({name: 'Bob'})]) // Will execute all queries in parallel

The exec function

From Mongoose docsIf you need a fully-fledged promise, use the .exec() function.

User.find({name: 'John'}).exec(); // Will execute returning a promise