🌱mongoDB:: population

Cross Database Populate

Let's say you have a schema representing events, and a schema representing conversations. Each event has a corresponding conversation thread.

const db1 = mongoose.createConnection('mongodb://localhost:27000/db1');
const db2 = mongoose.createConnection('mongodb://localhost:27001/db2');

const conversationSchema = new Schema({ numMessages: Number });
const Conversation = db2.model('Conversation', conversationSchema);

const eventSchema = new Schema({
  name: String,
  conversation: {
    type: ObjectId,
    ref: Conversation // `ref` is a **Model class**, not a string
  }
});
const Event = db1.model('Event', eventSchema);

In the above example, events and conversations are stored in separate MongoDB databases. String ref will not work in this situation, because Mongoose assumes a string ref refers to a model name on the same connection. In the above example, the conversation model is registered on db2, not db1.

// Works
const events = await Event.
  find().
  populate('conversation');

This is known as a "cross-database populate," because it enables you to populate across MongoDB databases and even across MongoDB instances.

If you don't have access to the model instance when defining your eventSchema, you can also pass the model instance as an option to populate().

const events = await Event.
  find().
  // The `model` option specifies the model to use for populating.
  populate({ path: 'conversation', model: Conversation });

 

Dynamic reference via 'refPath'

Mongoose can also populate from multiple collections based on the value of a property in the document. Let's say you're building a schema for storing comments. A user may comment on either a blog post or a product.

const commentSchema = new Schema({
  body: { type: String, required: true },
  doc: {
    type: Schema.Types.ObjectId,
    required: true,
    // Instead of a hardcoded model name in `ref`, `refPath` means Mongoose
    // will look at the `onModel` property to find the right model.
    refPath: 'docModel'
  },
  docModel: {
    type: String,
    required: true,
    enum: ['BlogPost', 'Product']
  }
});

const Product = mongoose.model('Product', new Schema({ name: String }));
const BlogPost = mongoose.model('BlogPost', new Schema({ title: String }));
const Comment = mongoose.model('Comment', commentSchema);

The refPath option is a more sophisticated alternative to ref. If ref is a string, Mongoose will always query the same model to find the populated subdocs. With refPath, you can configure what model Mongoose uses for each document.

const book = await Product.create({ name: 'The Count of Monte Cristo' });
const post = await BlogPost.create({ title: 'Top 10 French Novels' });

const commentOnBook = await Comment.create({
  body: 'Great read',
  doc: book._id,
  docModel: 'Product'
});

const commentOnPost = await Comment.create({
  body: 'Very informative',
  doc: post._id,
  docModel: 'BlogPost'
});

// The below `populate()` works even though one comment references the
// 'Product' collection and the other references the 'BlogPost' collection.
const comments = await Comment.find().populate('doc').sort({ body: 1 });
comments[0].doc.name; // "The Count of Monte Cristo"
comments[1].doc.title; // "Top 10 French Novels"

An alternative approach is to define separate blogPost and product properties on commentSchema, and then populate() on both properties.

const commentSchema = new Schema({
  body: { type: String, required: true },
  product: {
    type: Schema.Types.ObjectId,
    required: true,
    ref: 'Product'
  },
  blogPost: {
    type: Schema.Types.ObjectId,
    required: true,
    ref: 'BlogPost'
  }
});

// ...

// The below `populate()` is equivalent to the `refPath` approach, you
// just need to make sure you `populate()` both `product` and `blogPost`.
const comments = await Comment.find().
  populate('product').
  populate('blogPost').
  sort({ body: 1 });
comments[0].product.name; // "The Count of Monte Cristo"
comments[1].blogPost.title; // "Top 10 French Novels"

Defining separate blogPost and product properties works for this simple example. But, if you decide to allow users to also comment on articles or other comments, you'll need to add more properties to your schema. You'll also need an extra populate() call for every property, unless you use mongoose-autopopulate. Using refPath means you only need 2 schema paths and one populate() call regardless of how many models your commentSchema can point to.