set lean transform at schema level

[y-nk]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

I have found in issues and PR that .lean() supports a transform: (obj: any) => void function which is used to mutate the object retrieved from db post lean. The problem being that we need to put this function in every .lean() call.

I think it'd be nicer if we could have a leanTransform: (obj: any) => void option saved into the schema to avoid spreading the same transform function (or rather, set as base), similarly to what already exists for toObject and toJSON (schema.options.toObject.transform)

[vkarpov15]

The recommended way to do this is via middleware, schema.pre(['find', 'findOne', 'findOneAndUpdate', 'findOneAndReplace', 'findOneAndDelete']). Check for lean === true, and, if so, apply lean transform as follows:

  mongoose.plugin(schema => schema.pre(['find', 'findOne', 'findOneAndUpdate', 'findOneAndReplace', 'findOneAndDelete'], function() {
    if (this.mongooseOptions().lean === true) {
      this.lean({
        transform: (doc) => {
          doc.transformed = true;
          return doc;
        }
      });
    }
  }));

The above will apply your transform for all schemas, but you can also apply that middleware to individual schemas. Does this help?

[y-nk]

@vkarpov15 this does help indeed :) tho typewise it's a bit tricky and overall i feel like it's a bit weird that there are transform defaults for some methods (toObject/toJSON) but not all of them / at the same time i understand that the api could be clogged if too many options as well.

any opinion against this as options?

[vkarpov15]

Yeah I think it makes sense to support default lean options in schema options, maybe something like the following?

const schema = new Schema({ /* schema definition */ }, {
  lean: {
    transform: (doc) => {
          doc.transformed = true;
          return doc;
        }
  }
});

That way, if you apply lean() on a query, Mongoose would apply your lean options by default.

[y-nk]

@vkarpov15 yes it could be done at schema level, it makes more sense indeed.