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.

  1. const childSchema = new Schema({ name: 'string' });
  3. const parentSchema = new Schema({
  4.   // Array of subdocuments
  5.   children: [childSchema],
  6.   // Single nested subdocuments. Caveat: single nested subdocs only work
  7.   // in mongoose >= 4.2.0
  8.   child: childSchema
  9. });

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.

  1. const Parent = mongoose.model('Parent', parentSchema);
  2. const parent = new Parent({ children: [{ name: 'Matt' }, { name: 'Sarah' }] })
  3. parent.children[0].name = 'Matthew';
  5. // `parent.children[0].save()` is a no-op, it triggers middleware but
  6. // does **not** actually save the subdocument. You need to save the parent
  7. // doc.
  8. 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.

  1. childSchema.pre('save', function (next) {
  2.   if ('invalid' == this.name) {
  3.     return next(new Error('#sadpanda'));
  4.   }
  5.   next();
  6. });
  8. const parent = new Parent({ children: [{ name: 'invalid' }] });
  9. parent.save(function (err) {
  10.   console.log(err.message) // #sadpanda
  11. });

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.

  1. // Below code will print out 1-4 in order
  2. const childSchema = new mongoose.Schema({ name: 'string' });
  4. childSchema.pre('validate', function(next) {
  5.   console.log('2');
  6.   next();
  7. });
  9. childSchema.pre('save', function(next) {
  10.   console.log('3');
  11.   next();
  12. });
  14. const parentSchema = new mongoose.Schema({
  15.   child: childSchema
  16. });
  18. parentSchema.pre('validate', function(next) {
  19.   console.log('1');
  20.   next();
  21. });
  23. parentSchema.pre('save', function(next) {
  24.   console.log('4');
  25.   next();
  26. });

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.

  1. // Subdocument
  2. const subdocumentSchema = new mongoose.Schema({
  3.   child: new mongoose.Schema({ name: String, age: Number })
  4. });
  5. const Subdoc = mongoose.model('Subdoc', subdocumentSchema);
  7. // Nested path
  8. const nestedSchema = new mongoose.Schema({
  9.   child: { name: String, age: Number }
  10. });
  11. 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.

  1. const doc1 = new Subdoc({});
  2. doc1.child === undefined; // true
  3. doc1.child.name = 'test'; // Throws TypeError: cannot read property...
  5. const doc2 = new Nested({});
  6. doc2.child === undefined; // false
  7. console.log(doc2.child); // Prints 'MongooseDocument { undefined }'
  8. 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.

  1. const subdocumentSchema = new mongoose.Schema({
  2.   child: new mongoose.Schema({
  3.     name: String,
  4.     age: {
  5.       type: Number,
  6.       default: 0
  7.     }
  8.   })
  9. });
  10. const Subdoc = mongoose.model('Subdoc', subdocumentSchema);
  12. // Note that the `age` default has no effect, because `child`
  13. // is `undefined`.
  14. const doc = new Subdoc();
  15. doc.child; // undefined

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

  1. doc.child = {};
  2. // Mongoose applies the `age` default:
  3. 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.

  1. const childSchema = new mongoose.Schema({
  2.   name: String,
  3.   age: {
  4.     type: Number,
  5.     default: 0
  6.   }
  7. });
  8. const subdocumentSchema = new mongoose.Schema({
  9.   child: {
  10.     type: childSchema,
  11.     default: () => ({})
  12.   }
  13. });
  14. const Subdoc = mongoose.model('Subdoc', subdocumentSchema);
  16. // Note that Mongoose sets `age` to its default value 0, because
  17. // `child` defaults to an empty object and Mongoose applies
  18. // defaults to that empty object.
  19. const doc = new Subdoc();
  20. 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.

  1. 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:

  1. const Parent = mongoose.model('Parent');
  2. const parent = new Parent;
  4. // create a comment
  5. parent.children.push({ name: 'Liesl' });
  6. const subdoc = parent.children[0];
  7. console.log(subdoc) // { _id: '501d86090d371bab2c0341c5', name: 'Liesl' }
  8. subdoc.isNew; // true
  10. parent.save(function (err) {
  11.   if (err) return handleError(err)
  12.   console.log('Success!');
  13. });

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

  1. 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.

  1. // Equivalent to `parent.children.pull(_id)`
  2. parent.children.id(_id).remove();
  3. // Equivalent to `parent.child = null`
  4. parent.child.remove();
  5. parent.save(function (err) {
  6.   if (err) return handleError(err);
  7.   console.log('the subdocs were removed');
  8. });

Parents of Subdocs

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

  1. const schema = new Schema({
  2.   docArr: [{ name: String }],
  3.   singleNested: new Schema({ name: String })
  4. });
  5. const Model = mongoose.model('Test', schema);
  7. const doc = new Model({
  8.   docArr: [{ name: 'foo' }],
  9.   singleNested: { name: 'bar' }
  10. });
  12. doc.singleNested.parent() === doc; // true
  13. doc.docArr[0].parent() === doc; // true

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

  1. const schema = new Schema({
  2.   level1: new Schema({
  3.     level2: new Schema({
  4.       test: String
  5.     })
  6.   })
  7. });
  8. const Model = mongoose.model('Test', schema);
  10. const doc = new Model({ level1: { level2: 'test' } });
  12. doc.level1.level2.parent() === doc; // false
  13. doc.level1.level2.parent() === doc.level1; // true
  14. 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:

  1. const parentSchema = new Schema({
  2.   children: [{ name: 'string' }]
  3. });
  4. // Equivalent
  5. const parentSchema = new Schema({
  6.   children: [new Schema({ name: 'string' })]
  7. });

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.

  1. const schema = new Schema({
  2.   nested: {
  3.     prop: String
  4.   }
  5. });

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

  1. const schema = new Schema({
  2.   nested: {
  3.     // Do not do this! This makes `nested` a mixed path in Mongoose 5
  4.     type: { prop: String },
  5.     required: true
  6.   }
  7. });
  9. const schema = new Schema({
  10.   nested: {
  11.     // This works correctly
  12.     type: new Schema({ prop: String }),
  13.     required: true
  14.   }
  15. });

Next Up

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