import mongoose from 'mongoose';

const nestedSchema = new mongoose.Schema({
    targetType: String,
    targetId: mongoose.Schema.Types.ObjectId,
}, {
    virtuals: {
        target: {
            options: {
                ref: (doc: any) => {
                    // doc should be the sub document
                    // but instead, it is the parent document
                    console.debug(doc);
                    return doc.targetType;
                },
                localField: 'targetId',
                foreignField: '_id',
                justOne: true,
            }
        }
    }
});

const parentSchema = new mongoose.Schema({
    nested: {
        type: nestedSchema
    },
});

const NestedDynamicVirtualPopulateTest = mongoose.model('NestedDynamicVirtualPopulateTest', parentSchema);
async function run() {
  await mongoose.connect('mongodb://localhost:27017');
  await mongoose.connection.dropDatabase();
  const target = new NestedDynamicVirtualPopulateTest({});
  await target.save();

  const parent = new NestedDynamicVirtualPopulateTest({
      nested: {
          targetType: (target.constructor as typeof NestedDynamicVirtualPopulateTest).modelName,
          targetId: target._id,
      }
  });
  await parent.save();
  await parent.populate('nested.target');
  console.log(parent);
  console.log('=============');
  console.log(parent.nested);
  console.log('=================');
  console.log(parent.nested.target);
  console.log('======================')
}

run();