Mongoose-refPath-aggregation-workaround.js

const mongoose = require('mongoose')

const User = mongoose.model('User', new mongoose.Schema({
  name: String
}))

const Post = mongoose.model('Post', new mongoose.Schema({
  title: String
}))

const Comment = mongoose.model('Comment', new mongoose.Schema({
  content: String,
  target: {
    type: Schema.Types.ObjectId,
    required: true,
    refPath: 'targetModel'
  },
  targetModel: {
    type: String,
    required: true,
    enum: ['User', 'Post']
  }
}))

// populate dynamic reference via mongoose `refPath`                            
await Comment.find({ query }).populate('target').exec()

// populate dynamic reference via aggregation pipeline
const generatePipeline = ({ query, model, ref, refPath }) => [
  {
    $match: { ...query, [refPath]: model.modelName }
  },
  {
    $lookup: {
      from: model.collection.name,
      foreignField: '_id',
      localField: ref,
      as: ref
    }
  },
  {
    $addFields: {
      [ref]: { $arrayElemAt: [`$${ref}`, 0] }
    }
  }
]
await Comment.aggregate(
  [User, Post].flatMap((model, index) => {
    const pipeline = generatePipeline({ query, model, ref: 'target', refPath: 'targetModel' })
    return index === 0 ? pipeline : [{ $unionWith: { coll: Comment.collection.name, pipeline } }]
  })
).exec()