Fix regression for overwriteModels flag

[chochihim]

Below code throws MissingSchemaError in the populate statement:

const mongoose = require("mongoose");

const { Schema } = mongoose;

async function run() {
  try {
    await mongoose.connect(process.env.MONGODB_URI);

    mongoose.set("overwriteModels", true);

    const personSchema = Schema({
      name: String,
    });

    const storySchema = Schema({
      author: { type: Schema.Types.ObjectId, ref: "Person" },
      title: String,
    });

    const Story = mongoose.model("Story", storySchema);
    const Person = mongoose.model("Person", personSchema);

    const person = await Person.create({ name: "David" });
    await Story.create({
      title: "My Story",
      author: person._id,
    });

    await Story.findOne({ title: "My Story" })
      .populate("author")
      .exec();
  } catch (error) {
    console.error(error);
  }
}

run();

It will works if I do not set overwriteModels as true.

I find that it is caused by 6865fea. Looks like that commit prevents mixing models registered in non-default connection and global mongoose instance.

How about adding back the line for model lookup but lookup from the connection itself (lookup from this.models instead of this.base.models)?

Summary

Examples

[vkarpov15]

This works, but I think the more correct fix would be to just return this.models[name] if calling this method with 1 argument. What do you think?