Mongoose with typescript?

Question

I have a project in nodejs and typescript. I'm using mongoose to connect to a mongoDb database. My code looks like this

import { Schema, Document, Model } from 'mongoose';
import * as mongoose from 'mongoose';

export interface IProblem extends Document {
  problem: string;
  solution: string;
}

const ProblemSchema = new Schema({
  problem: { type: String, required: true },
  solution: { type: String, required: true },
});

export async function findOneByProblem(
  this: IProblemModel,
  { problem, solution }: { problem: string; solution: string }
): Promise<IProblem> {
  const record = await this.findOne({ problem, solution });
  return record;
}

export default mongoose.model('Problem', ProblemSchema);

ProblemSchema.statics.findOneByProblem = findOneByProblem;

export interface IProblemModel extends Model<IProblem> {
  findOneByProblem: (
    this: IProblemModel,
    { problem, solution }: { problem: string; solution: string }
  ) => Promise<IProblem>;
}

However, at these lines

const record = await this.findOne({ problem, solution });
return record;

I get a compiler error saying this

TS2322: Type 'IProblem | null' is not assignable to type 'IProblem'.   Type 'null' is not assignable to type 'IProblem'.

Am I missing something?

Answer 1

Your type for findOneByProblem is wrong – after all, it's possible that you don't find an IProblem instance, and the result is null.

The correct type is

Promise<IProblem | null>

– or you could internally if(problem === null) throw new Error("No Problem found"); or similar in the function if you don't want to change the type.