Populate with TypeScript

Mongoose's TypeScript bindings add a generic parameter Paths to the populate():

import { Schema, model, Document, Types } from 'mongoose';

// `Parent` represents the object as it is stored in MongoDB
interface Parent {
  child?: Types.ObjectId,
  name?: string
}
const ParentModel = model<Parent>('Parent', new Schema({
  child: { type: Schema.Types.ObjectId, ref: 'Child' },
  name: String
}));

interface Child {
  name: string;
}
const childSchema = new Schema({ name: String });
const ChildModel = model<Child>('Child', childSchema);

// Populate with `Paths` generic `{ child: Child }` to override `child` path
ParentModel.findOne({}).populate<{ child: Child }>('child').orFail().then(doc => {
  // Works
  const t: string = doc.child.name;
});

An alternative approach is to define a PopulatedParent interface and use Pick<> to pull the properties you're populating.

import { Schema, model, Document, Types } from 'mongoose';

// `Parent` represents the object as it is stored in MongoDB
interface Parent {
  child?: Types.ObjectId,
  name?: string
}
interface Child {
  name: string;
}
interface PopulatedParent {
  child: Child | null;
}
const ParentModel = model<Parent>('Parent', new Schema({
  child: { type: Schema.Types.ObjectId, ref: 'Child' },
  name: String
}));
const childSchema = new Schema({ name: String });
const ChildModel = model<Child>('Child', childSchema);

// Populate with `Paths` generic `{ child: Child }` to override `child` path
ParentModel.findOne({}).populate<Pick<PopulatedParent, 'child'>>('child').orFail().then(doc => {
  // Works
  const t: string = doc.child.name;
});

Using PopulatedDoc

Mongoose also exports a PopulatedDoc type that helps you define populated documents in your document interface:

import { Schema, model, Document, PopulatedDoc } from 'mongoose';

// `child` is either an ObjectId or a populated document
interface Parent {
  child?: PopulatedDoc<Document<ObjectId> & Child>,
  name?: string
}
const ParentModel = model<Parent>('Parent', new Schema({
  child: { type: 'ObjectId', ref: 'Child' },
  name: String
}));

interface Child {
  name?: string;
}
const childSchema = new Schema({ name: String });
const ChildModel = model<Child>('Child', childSchema);

ParentModel.findOne({}).populate('child').orFail().then((doc: Parent) => {
  const child = doc.child;
  if (child == null || child instanceof ObjectId) {
    throw new Error('should be populated');
  } else {
    // Works
    doc.child.name.trim();
  }
});

However, we recommend using the .populate<{ child: Child }> syntax from the first section instead of PopulatedDoc. Here's two reasons why:

1. You still need to add an extra check to check if child instanceof ObjectId. Otherwise, the TypeScript compiler will fail with Property name does not exist on type ObjectId. So using PopulatedDoc<> means you need an extra check everywhere you use doc.child.
2. In the Parent interface, child is a hydrated document, which makes it difficult for Mongoose to infer the type of child when you use lean() or toObject().