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
  child: childSchema
});

Note that populated documents are not subdocuments in Mongoose. Subdocument data is embedded in the top-level document. Referenced documents are separate top-level documents.

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

const parentSchema = new Schema({
  child: {
    type: mongoose.ObjectId,
    ref: 'Child'
  }
});
const Parent = mongoose.model('Parent', parentSchema);

const doc = await Parent.findOne().populate('child');
// NOT a subdocument. `doc.child` is a separate top-level document.
doc.child;

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.
await parent.save();

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' }] });
try {
  await parent.save();
} catch (err) {
  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

Subdocument Defaults

Subdocument paths are undefined by default, and Mongoose does not apply subdocument defaults unless you set the subdocument path to a non-nullish value.

const subdocumentSchema = new mongoose.Schema({
  child: new mongoose.Schema({
    name: String,
    age: {
      type: Number,
      default: 0
    }
  })
});
const Subdoc = mongoose.model('Subdoc', subdocumentSchema);

// Note that the `age` default has no effect, because `child`
// is `undefined`.
const doc = new Subdoc();
doc.child; // undefined

However, if you set doc.child to any object, Mongoose will apply the age default if necessary.

doc.child = {};
// Mongoose applies the `age` default:
doc.child.age; // 0

Mongoose applies defaults recursively, which means there's a nice workaround if you want to make sure Mongoose applies subdocument defaults: make the subdocument path default to an empty object.

const childSchema = new mongoose.Schema({
  name: String,
  age: {
    type: Number,
    default: 0
  }
});
const subdocumentSchema = new mongoose.Schema({
  child: {
    type: childSchema,
    default: () => ({})
  }
});
const Subdoc = mongoose.model('Subdoc', subdocumentSchema);

// Note that Mongoose sets `age` to its default value 0, because
// `child` defaults to an empty object and Mongoose applies
// defaults to that empty object.
const doc = new Subdoc();
doc.child; // { age: 0 }

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

await parent.save();
console.log('Success!');

You can also create a subdocument without adding it to an array by using the create() method of Document Arrays.

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

Removing Subdocs

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

// Equivalent to `parent.children.pull(_id)`
parent.children.id(_id).deleteOne();
// Equivalent to `parent.child = null`
parent.child.deleteOne();

await parent.save();
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' })]
});

Next Up

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

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