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: 'ObjectId', ref: 'Child' },
  name: String
}));

interface Child {
  name: string;
}
const childSchema: Schema = 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: 'ObjectId', ref: 'Child' },
  name: String
}));
const childSchema: Schema = 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;
});