text: const mongoose = require('mongoose')
  const Schema = mongoose.Schema
  const ObjectId = mongoose.Types.ObjectId
  
  const FollowSchema = new Schema(
    {
      userId: { type: ObjectId, required: true },
      followId: { type: ObjectId, required: true }
    },
    {
      versionKey: false
    }
  )
  FollowSchema.index({ userId: 1, followId: 1 }, { unique: true })
  
  exports.Follow = mongoose.model('Follow', FollowSchema)
  
  exports.validateObjectId = idString => {
    return ObjectId.isValid(idString)
  }
  