When executing a query what a callback function should be

摘自mongoose官网   

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 query builder interface.

In mongoose 4, a Query has a .then() function, and thus can be used as a promise.

When executing a query with a callback function, you specify your query as a JSON document. The JSON document's syntax is the same as the MongoDB shell.

var Person = mongoose.model('Person', yourSchema);// find each person with a last name matching 'Ghost', selecting the `name` and `occupation` fieldsPerson.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 errorparameter will contain an error document, and result will be null. If the query is successful, the errorparameter 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 patterncallback(error, result). What result 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() thenumber of documents affected, etc. The API docs for Models provide more detail on what is passed to the callbacks.