// mongoose schema
const userSchema = new Schema({
  name: String,
  friendIds: [String],
})

userSchema.methods.friends = function() {
  return User.where("_id").in(this.friendIds)
}

const User = mongoose.model("User", userSchema)

// GraphQL schema
const graphqlSchema = `
  type User {
    id: ID!
    name: String
    friends: [User]
  }

  type Query {
    users: [User]
  }
`

// GraphQL resolver
const resolver = {
  Query: {
    users: () => User.find()
  }
}

[
  { id: 1, name: "Alice", friendIds: [2, 3] },
  { id: 2, name: "Bob", friendIds: [1, 3] },
  { id: 3, name: "Charlie", friendIds: [2, 4, 5] },
  { id: 4, name: "David", friendIds: [1, 5] },
  { id: 5, name: "Ethan", friendIds: [1, 4, 2] },
]

{
  users {
    name
    friends {
      name
    }
  }
}

// mongoose schema
const userSchema = new Schema({
  name: String,
  friendIds: [String]
})

userSchema.methods.friends = function() {
  return userLoader.load(this.friendIds)
}

const User = mongoose.model("User", userSchema)

const userLoader = new Dataloader(userIds => {
  const users = await User.where("_id").in(userIds)
  const usersMap = new Map(users.map(user => [user.id, user]))
  return userIds.map(userId => usersMap.get(userId))
})

// GraphQL resolver
const resolver = {
  Query: {
    users: async () => {
      const userLoader = new Dataloader(userIds => {
        const users = await User.where("_id").in(userIds)
        const usersMap = new Map(users.map(user => [user.id, user]))
        return userIds.map(userId => usersMap.get(userId))
      })
      const userIds = await User.find().distinct("_id")
      return userLoader.load(userIds)
    }
  }
}

// GraphQL resolver
const resolver = {
  Query: {
    users: async () => {
      const userLoader = new Dataloader(userIds => {
        const users = await User.where("_id").in(userIds)
        const usersMap = new Map(users.map(user => [user.id, user]))
        return userIds.map(userId => usersMap.get(userId))
      })
      const userSchema = new Schema({
        name: String,
        friendIds: [String]
      })
      userSchema.methods.friends = function() {
        return userLoader.load(this.friendIds)
      }
      const User = mongoose.model("User", userSchema)
      const userIds = await User.find().distinct("_id")
      return userLoader.load(userIds)
    }
  }
}