Queries

Documents can be retrieved through several static helper methods of models.

Any model method which involves specifying query conditions can be executed two ways:

When a callback function:

• is passed, the operation will be executed immediately with the results passed to the callback.
• is not passed, an instance of Query is returned, which provides a special QueryBuilder interface for you.

Let’s take a look at what happens when passing a callback:

var Person = mongoose.model('Person', yourSchema);
// find each person with a last name matching 'Ghost', selecting the `name` and `occupation` fields
Person.findOne({ 'name.last': 'Ghost' }, 'name occupation', function (err, person) {
  if (err) return handleError(err);
  console.log('%s %s is a %s.', person.name.first, person.name.last, person.occupation) // Space Ghost is a talk show host.
})

Here we see that the query was executed immediately and the results passed to our callback. All callbacks in Mongoose use the pattern: callback(error, result). If an error occurs executing the query, the error parameter will contain an error document, and result will be null. If the query is successful, the error parameter will be null, and the result will be populated with the results of the query.

Anywhere a callback is passed to a query in Mongoose, the callback follows the pattern callback(error, results). What results is depends on the operation: For findOne() it is a potentially-null single document, find() a list of documents, count() the number of documents, update() the number of documents affected, etc. The API docs for Models provide more detail on what is passed to the callbacks.

Now let’s look at what happens when no callback is passed:

// find each person with a last name matching 'Ghost'
var query = Person.findOne({ 'name.last': 'Ghost' });
// selecting the `name` and `occupation` fields
query.select('name occupation');
// execute the query at a later time
query.exec(function (err, person) {
  if (err) return handleError(err);
  console.log('%s %s is a %s.', person.name.first, person.name.last, person.occupation) // Space Ghost is a talk show host.
})

An instance of Query was returned which allows us to build up our query. Taking this example further:

Person
.find({ occupation: /host/ })
.where('name.last').equals('Ghost')
.where('age').gt(17).lt(66)
.where('likes').in(['vaporizing', 'talking'])
.limit(10)
.sort('-occupation')
.select('name occupation')
.exec(callback);

References to other documents

There are no joins in MongoDB but sometimes we still want references to documents in other collections. This is where population comes in. Read more about how to include documents from other collections in your query results here.

Streaming

Queries can be streamed from MongoDB to your application as well. Simply call the query’s stream method instead of exec to return an instance of QueryStream. Note: QueryStreams are Node.js 0.8 style read streams not Node.js 0.10 style.

Next Up

Now that we’ve covered Queries, let’s take a look at validation.