Sub Docs

Subdocuments are documents embedded in other documents. In Mongoose, this means you can nest schemas in other schemas. Mongoose has two distinct notions of subdocuments: arrays of subdocuments and single nested subdocuments.

var childSchema = new Schema({ name: 'string' });

var parentSchema = new Schema({
  // Array of subdocuments
  children: [childSchema],
  // Single nested subdocuments. Caveat: single nested subdocs only work
  // in mongoose >= 4.2.0
  child: childSchema
});

Subdocuments are similar to normal documents. Nested schemas can have middleware, custom validation logic, virtuals, and any other feature top-level schemas can use. The major difference is that subdocuments are not saved individually, they are saved whenever their top-level parent document is saved.

var Parent = mongoose.model('Parent', parentSchema);
var parent = new Parent({ children: [{ name: 'Matt' }, { name: 'Sarah' }] })
parent.children[0].name = 'Matthew';

// `parent.children[0].save()` is a no-op, it triggers middleware but
// does **not** actually save the subdocument. You need to save the parent
// doc.
parent.save(callback);

Subdocuments have save and validate middleware just like top-level documents. Calling save() on the parent document triggers the save() middleware for all its subdocuments, and the same for validate() middleware.

childSchema.pre('save', function (next) {
  if ('invalid' == this.name) {
    return next(new Error('#sadpanda'));
  }
  next();
});

var parent = new Parent({ children: [{ name: 'invalid' }] });
parent.save(function (err) {
  console.log(err.message) // #sadpanda
});

Subdocuments' pre('save') and pre('validate') middleware execute before the top-level document's pre('save') but after the top-level document's pre('validate') middleware. This is because validating before save() is actually a piece of built-in middleware.

// Below code will print out 1-4 in order
var childSchema = new mongoose.Schema({ name: 'string' });

childSchema.pre('validate', function(next) {
  console.log('2');
  next();
});

childSchema.pre('save', function(next) {
  console.log('3');
  next();
});

var parentSchema = new mongoose.Schema({
  child: childSchema,
    });
    
parentSchema.pre('validate', function(next) {
  console.log('1');
  next();
});

parentSchema.pre('save', function(next) {
  console.log('4');
  next();
});

Finding a sub-document

Each subdocument has an _id by default. Mongoose document arrays have a special id method for searching a document array to find a document with a given _id.

var doc = parent.children.id(_id);

Adding sub-docs to arrays

MongooseArray methods such as push, unshift, addToSet, and others cast arguments to their proper types transparently:

var Parent = mongoose.model('Parent');
var parent = new Parent;

// create a comment
parent.children.push({ name: 'Liesl' });
var subdoc = parent.children[0];
console.log(subdoc) // { _id: '501d86090d371bab2c0341c5', name: 'Liesl' }
subdoc.isNew; // true

parent.save(function (err) {
  if (err) return handleError(err)
  console.log('Success!');
});

Sub-docs may also be created without adding them to the array by using the create method of MongooseArrays.

var newdoc = parent.children.create({ name: 'Aaron' });

Removing subdocs

Each subdocument has it's own remove method. For an array subdocument, this is equivalent to calling .pull() on the subdocument. For a single nested subdocument, remove() is equivalent to setting the subdocument to null.

// Equivalent to `parent.children.pull(_id)`
parent.children.id(_id).remove();
// Equivalent to `parent.child = null`
parent.child.remove();
parent.save(function (err) {
  if (err) return handleError(err);
  console.log('the subdocs were removed');
});

Alternate declaration syntax for arrays

If you create a schema with an array of objects, mongoose will automatically convert the object to a schema for you:

var parentSchema = new Schema({
  children: [{ name: 'string' }]
});
// Equivalent
var parentSchema = new Schema({
  children: [new Schema({ name: 'string' })]
});

Next Up

Now that we've covered Sub-documents, let's take a look at querying.