How to write a generic function to resolve PopulatedDoc type variables?

[nikzanda]

Prerequisites

• I have written a descriptive issue title

Mongoose version

8.9.1

Node.js version

20.10.0

MongoDB version

6.0.2

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

No response

Issue

In my project, I often find myself having to implement this code:

// Check if 'myPopulatedDocProp' has already been populated in a previous part of the code
if (myDocument.myPopulatedDocProp instanceof AnotherModel) {
  return myDocument.myPopulatedDocProp;
}

// If 'myPopulatedDocProp' was not populated, perform a database query to find and populate the document property
const result = await AnotherModel
  .findById(myDocument.myPopulatedDocProp) // Query the AnotherModel to find the document by ID
  .orFail(); // Throw an error if the document is not found
return result; // Return the populated document property

Is it possible to write a generic function so that I don't have to rewrite the same exact code but with different models?
I tried with this code but it doesn't work:

export const findOrReturnInstance = async <T extends Document>(
  populatedDoc: PopulatedDoc<T>,
  model: Model<T>,
) => {
  if (populatedDoc instanceof model) {
    return populatedDoc;
  }
  const result = await model
    .findById(populatedDoc)
    .orFail();
  return result;
};

Is it the right approach or am I doing something wrong?
Attention, with this generic function I want to ensure that any virtual variables and instance methods, etc., are preserved.

Thank you for your help. 🙏🏻

[vkarpov15]

The approach is reasonable, you just need to type model correctly. I would also recommend against using PopulatedDoc<> type for reasons explained here.

Take a look at the following script, would this approach work for you?

import mongoose, { Schema } from 'mongoose';

const ParentModel = mongoose.model('Parent', new Schema({
  child: { type: Schema.Types.ObjectId, ref: 'Child' },
  name: String
}));

const childSchema: Schema = new Schema({ name: String });
const ChildModel = mongoose.model('Child', childSchema);
type ChildHydratedDocType = ReturnType<(typeof ChildModel)['hydrate']>;

(async function() {
  const doc = await ParentModel.findOne({}).populate<{ child: ChildHydratedDocType }>('child').orFail();
  const res1: ChildHydratedDocType = await findOrReturnInstance(doc.child, ChildModel);

  const doc2 = await ParentModel.findOne().orFail();
  const res2: ChildHydratedDocType = await findOrReturnInstance(doc.child, ChildModel);
})();

async function findOrReturnInstance<HydratedDocType extends mongoose.Document>(
  docOrId: HydratedDocType | mongoose.Types.ObjectId,
  Model: mongoose.Model<any, any, any, any, HydratedDocType>
) {
  if (docOrId instanceof mongoose.Document) {
    return docOrId;
  }
  
  return Model.findById(docOrId).orFail();
}  

[nikzanda]

The script is fine, although I would have preferred the type to be inferred already by findOrReturnInstance and not specified manually.

I have another question: if I avoid using PopulatedDoc, how do I handle the following situation?

function fn(p: ParentInstance) {
   // TODO
}

const parent = await Parent.findOne({}).populate<{ child: ChildInstance }>('child').orFail();

// There appears to be a compilation error here, because if I populate child, it will no longer be of type ObjectId.
fn(parent); 

[vkarpov15]

Can you please elaborate on "although I would have preferred the type to be inferred already by findOrReturnInstance and not specified manually."?

Re: PopulatedDoc, that seems like expected behavior because parent is no longer strictly the same type as ParentInstance. You can always do something like fn(p: ParentInstance & { child: ChildInstance }) if you want to allow populated docs to be passed into fn().

[nikzanda]

Let me explain better: in the example you wrote, for res1 and res2 you explicitly declared their types. Do you think it is possible to make the findOrReturnInstance function return the correct type without having to specify it?