Hey all! I’m having trouble with Absinthe and Dataloader when trying to resolve friends of a user.

I was able to load the friends without much issue, but now I’m trying to add a status to the response, which lives on the join table.

The last chunk in the code below is where I’m struggling. Any insight is much appreciated! Thanks!

# User Ecto Schema
@primary_key {:id, :binary_id, autogenerate: true}
schema "users" do
  ...
  has_many :friends_assoc, Friend, foreign_key: :user_id
  has_many :reverse_friends_assoc, Friend, foreign_key: :friend_id
  has_many :friends, through: [:friends_assoc, :friend]
  has_many :reverse_friends, through: [:reverse_friends_assoc, :user]
end

# Friend Ecto Schema
@primary_key {:id, :binary_id, autogenerate: true}
schema "friends" do
  field :status, FriendStatus
  belongs_to :user, User, type: :binary_id
  belongs_to :friend, User, type: :binary_id
end

# User Absinthe Schema
object :user do
  field :id, non_null(:string)
  ...
  field :friends, list_of(:friend) do
    arg :status, :friend_status
    # This ultimately points to: `Dataloader.Ecto.new(Repo, query: fn _, args -> generate_query(args) end)`
    resolve dataloader(User)
  end
end

# Friend Absinthe Schema
object :friend do
  field :id, :string
  field :first_name, :string
  field :last_name, :string
  # This comes back as `null` on the GraphQL response
  # because it's not on the Ecto User schema 
  # but I'm not sure how to remedy the situation
  field :status, :friend_status
end

UPDATE

I wound up going with a custom Dataloader.KV that points to this function

@doc """
Queries the database for Users and loads their friends.
"""
@spec get_friends_with_status([Ecto.UUID.t()], map()) :: map()
def get_friends_with_status(ids, %{status: status} = _args) do
  query = from u in User,
    left_join: fa in assoc(u, :friends_assoc),
    left_join: fr in assoc(fa, :friend),
    where: u.id in ^ids,
    where: fa.status == ^status,
    select: %{
      user_id: u.id,
      friend: %{
        first_name: fr.first_name,
        last_name: fr.last_name,
        email: fr.email,
        id: fr.id,
        status: fa.status
      }
    }

  id_map = Map.new(ids, & {&1, []})

  query
  |> Repo.all()
  |> Enum.reduce(id_map, fn %{user_id: user_id, friend: friend}, acc ->
    Map.update(acc, user_id, [friend], fn
      friends -> [friend | friends]
    end)
  end)
end