What is the type of Mongoose populate in TypeScript before using it in Express?

I am in the process of upgrading an outdated JavaScript Express application to a new TypeScript application. I have encountered an issue with typing during the populate process.

/* This function acts like middleware after creating our schema, allowing us to access 'next' */
/* It needs to be a function declaration (not an arrow function) so we can use 'this' to refer to our schema */
const autoPopulatePostedBy = function (next) {
  this.populate('postedBy', '_id username avatar');
  this.populate('comments.postedBy', '_id username avatar');
  next();
};

/* We need to populate the 'postedBy' field almost every time we perform a findOne/find query, so we'll set it up as a pre hook when creating the schema */
postSchema.pre<IPostSchema>('findOne', autoPopulatePostedBy).pre<IPostSchema>('find', autoPopulatePostedBy);
/* Create index on keys for more performant querying/post sorting */

postSchema.index({ postedBy: 1, createdAt: 1 });

export default model<IPostSchema>('Post', postSchema);

I am facing errors related to 'next' and 'this', TypeScript is indicating types as any and for pre.

Argument of type '"findOne"' is not assignable to parameter of type 'RegExp | MongooseDocumentMiddleware | MongooseDocumentMiddleware[]'

How can I resolve this issue?

Answer №1

To properly use the pre hooks, make sure the parameterized type is as follows:

Query<ResultType, Doctype extends Document<any, {}>, THelpers={}>

The type you've specified for your IPostSchema should work for both the Doctype and ResultType in this case.

For example:

postSchema.pre<Query<IPostInterfaceDocument, IPostInterfaceDocument>>('findOne', .....

Don't forget to import Query from mongoose.

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