How to Use Discriminator in Mongoose for Schema Inheritance

In Mongoose, discriminator allows you to create multiple models with a shared base schema but different specialized schemas. You define a base schema, then use Model.discriminator() to create child models that inherit the base schema and add their own fields.

Syntax

The discriminator method is called on a base model to create a new model with an extended schema. It takes two main arguments: the name of the new model and the schema that extends the base schema.

• Base Schema: The common fields shared by all models.
• Discriminator Name: A string to identify the child model.
• Discriminator Schema: The schema with additional fields specific to the child model.

javascript

const baseSchema = new mongoose.Schema({
  commonField: String
});

const BaseModel = mongoose.model('Base', baseSchema);

const childSchema = new mongoose.Schema({
  specificField: Number
});

const ChildModel = BaseModel.discriminator('Child', childSchema);

Example

This example shows how to create a base Event model and two discriminators: ClickedLinkEvent and SignedUpEvent. Each child model has its own specific fields but shares the base schema fields.

javascript

const mongoose = require('mongoose');

mongoose.connect('mongodb://localhost:27017/testdb', { useNewUrlParser: true, useUnifiedTopology: true });

const eventSchema = new mongoose.Schema({
  timestamp: Date,
  userId: String
});

const Event = mongoose.model('Event', eventSchema);

const clickedLinkSchema = new mongoose.Schema({
  url: String
});

const signedUpSchema = new mongoose.Schema({
  plan: String
});

const ClickedLinkEvent = Event.discriminator('ClickedLink', clickedLinkSchema);
const SignedUpEvent = Event.discriminator('SignedUp', signedUpSchema);

async function run() {
  await mongoose.connection.dropDatabase();

  const click = new ClickedLinkEvent({
    timestamp: new Date(),
    userId: 'user123',
    url: 'https://example.com'
  });

  const signup = new SignedUpEvent({
    timestamp: new Date(),
    userId: 'user456',
    plan: 'Pro'
  });

  await click.save();
  await signup.save();

  const events = await Event.find();
  console.log(events);

  mongoose.connection.close();
}

run();

Output

[
  {
    _id: ObjectId("..."),
    timestamp: 2024-06-01T00:00:00.000Z,
    userId: 'user123',
    url: 'https://example.com',
    __t: 'ClickedLink',
    __v: 0
  },
  {
    _id: ObjectId("..."),
    timestamp: 2024-06-01T00:00:00.000Z,
    userId: 'user456',
    plan: 'Pro',
    __t: 'SignedUp',
    __v: 0
  }
]

Common Pitfalls

• Not defining a base model: You must create a base model before using discriminator.
• Schema conflicts: Avoid overlapping field names in base and discriminator schemas.
• Missing __t field: Mongoose adds a __t field to identify the discriminator type; do not remove or override it.
• Using discriminators with different collections: Discriminators share the same MongoDB collection as the base model.

javascript

/* Wrong: Using discriminator without base model */
// const Child = mongoose.model('Child', new mongoose.Schema({ field: String }));
// const Wrong = Child.discriminator('Wrong', new mongoose.Schema({ extra: Number }));

/* Right: Define base model first */
const Base = mongoose.model('Base', new mongoose.Schema({ field: String }));
const Correct = Base.discriminator('Correct', new mongoose.Schema({ extra: Number }));

Quick Reference

Discriminator Cheat Sheet:

Step	Action
1	Create base schema and model
2	Define child schemas with extra fields
3	Use BaseModel.discriminator(name, schema) to create child models
4	Save and query using base or child models

Key Takeaways

Discriminator lets you create related models sharing a base schema with extra fields.

Always define a base model before creating discriminators.

Discriminators share the same MongoDB collection as the base model.

Mongoose adds a __t field to track the discriminator type automatically.

Avoid field name conflicts between base and discriminator schemas.