Subdocuments

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.

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

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

Aside from code reuse, one important reason to use subdocuments is to create a path where there would otherwise not be one to allow for validation over a group of fields (e.g. dateRange.fromDate <= dateRange.toDate).

What is a Subdocument?

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.

const Parent = mongoose.model('Parent', parentSchema);
const 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();
});

const 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
const childSchema = new mongoose.Schema({ name: 'string' });

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

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

const parentSchema = new mongoose.Schema({
  child: childSchema
});

parentSchema.pre('validate', function(next) {
  console.log('1');
  next();
});

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

Subdocuments versus Nested Paths

In Mongoose, nested paths are subtly different from subdocuments. For example, below are two schemas: one with child as a subdocument, and one with child as a nested path.

// Subdocument
const subdocumentSchema = new mongoose.Schema({
  child: new mongoose.Schema({ name: String, age: Number })
});
const Subdoc = mongoose.model('Subdoc', subdocumentSchema);

// Nested path
const nestedSchema = new mongoose.Schema({
  child: { name: String, age: Number }
});
const Nested = mongoose.model('Nested', nestedSchema);

These two schemas look similar, and the documents in MongoDB will have the same structure with both schemas. But there are a few Mongoose-specific differences:

First, instances of Nested never have child === undefined. You can always set subproperties of child, even if you don't set the child property. But instances of Subdoc can have child === undefined.

const doc1 = new Subdoc({});
doc1.child === undefined; // true
doc1.child.name = 'test'; // Throws TypeError: cannot read property...

const doc2 = new Nested({});
doc2.child === undefined; // false
console.log(doc2.child); // Prints 'MongooseDocument { undefined }'
doc2.child.name = 'test'; // Works

Secondly, in Mongoose 5, Document#set() merges when you call it on a nested path, but overwrites when you call it on a subdocument.

const doc1 = new Subdoc({ child: { name: 'Luke', age: 19 } });
doc1.set({ child: { age: 21 } });
doc1.child; // { age: 21 }

const doc2 = new Nested({ child: { name: 'Luke', age: 19 } });
doc2.set({ child: { age: 21 } });
doc2.child; // { name: Luke, age: 21 }

Finding a Subdocument

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.

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

Adding Subdocs to Arrays

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

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

// create a comment
parent.children.push({ name: 'Liesl' });
const 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!');
});

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

const 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');
});

Parents of Subdocs

Sometimes, you need to get the parent of a subdoc. You can access the parent using the parent() function.

const schema = new Schema({
  docArr: [{ name: String }],
  singleNested: new Schema({ name: String })
});
const Model = mongoose.model('Test', schema);

const doc = new Model({
  docArr: [{ name: 'foo' }],
  singleNested: { name: 'bar' }
});

doc.singleNested.parent() === doc; // true
doc.docArr[0].parent() === doc; // true

If you have a deeply nested subdoc, you can access the top-level document using the ownerDocument() function.

const schema = new Schema({
  level1: new Schema({
    level2: new Schema({
      test: String
    })
  })
});
const Model = mongoose.model('Test', schema);

const doc = new Model({ level1: { level2: 'test' } });

doc.level1.level2.parent() === doc; // false
doc.level1.level2.parent() === doc.level1; // true
doc.level1.level2.ownerDocument() === doc; // true

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:

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

Alternate declaration syntax for single nested subdocuments

Unlike document arrays, Mongoose 5 does not convert an objects in schemas into nested schemas. In the below example, nested is a nested path rather than a subdocument.

const schema = new Schema({
  nested: {
    prop: String
  }
});

This leads to some surprising behavior when you attempt to define a nested path with validators or getters/setters.

const schema = new Schema({
  nested: {
    // Do not do this! This makes `nested` a mixed path in Mongoose 5
    type: { prop: String },
    required: true
  }
});

const schema = new Schema({
  nested: {
    // This works correctly
    type: new Schema({ prop: String }),
    required: true
  }
});

Surprisingly, declaring nested with an object type makes nested into a path of type Mixed. To instead make Mongoose automatically convert type: { prop: String } into type: new Schema({ prop: String }), set the typePojoToMixed option to false.

const schema = new Schema({
  nested: {
    // Because of `typePojoToMixed`, Mongoose knows to
    // wrap `{ prop: String }` in a `new Schema()`.
    type: { prop: String },
    required: true
  }
}, { typePojoToMixed: false });

Next Up

Now that we've covered Subdocuments, let's take a look at querying.