Mongoose projection example

projection.ts

import { Schema, model } from 'mongoose';

interface IThing {
  field1: string;
  field2: string;
}

(async () => {
  const thingSchema = new Schema<IThing>({
    field1: String,
    field2: String,
  });

  const things = model('things', thingSchema);

  // The second parameter should be typed as ('field1' | 'field2')[] or similar, instead of `any`
  const proj = await things.findOne({}, ['field1', 'abc']);

  proj?.field1; // This is allowed correctly
  proj?.field2; // This should not be allowed, as the key will not be present due to the projection used
})();

workaround.ts

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

export type Projection<
  Schema extends {},
  Keys extends keyof Schema,
  Id = ObjectId
> = Omit<Schema, Exclude<keyof Schema, Keys>> & Document<Id>;

(async () => {
  interface IThing {
    field1: string;
    field2: string;
  }

  const thingSchema = new Schema<IThing>({
    field1: String,
    field2: String,
  });

  const things = model('things', thingSchema);

  const projectionKeys: (keyof IThing)[] = ['field1'];
  const proj = (await things.findOne({}, projectionKeys)) as Projection<IThing, 'field1'>;

  proj?.field1;
  // Doesn't allow proj.field2
})();