Dataloader on Join Table Association

Questions / Help
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1

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
2

Hello!
Personally I would go with another graphql schema.
:friend object is context-specific. It has an id of the user, but different status depending on parent record. This is error-prone, because GraphQL client on frontend side MAY use simple caching by id field only, so other users may borrow friends.
To avoid this you have to use an id from the friends table. But it would be simpler to expose :user association:

object :user do
  field :id, non_null(:string)
  ...
  field :friends, list_of(:friend) do
    arg :status, :friend_status
    # resolve using :friends_assoc
  end
end

object :friend do
  field :id, non_null(:string)
  field :status, :friend_status
  field :user, :user
  field :friend, :user
end
1 Like
3

Thanks for the suggestion! I would up going with this implementation