How do I use Absinthe Dataloader for many to many relationships - graphql

Question

In my phoenix application, I have a many to many relationship between an Artist and a Cause schema implemented using a join table artists_causes. In my Artist schema, I have many_to_many :causes, Cause, join_through: "artists_causes" and in the Cause schema I have many_to_many :artists, Artist, join_through: "artists_causes"
I am using absinthe for graphql and in my CauseTypes module, I have implemented a the cause object as below
defmodule MyAppWeb.Schema.CauseTypes do
#moduledoc """
All types for causes
"""
use Absinthe.Schema.Notation
import Absinthe.Resolution.Helpers, only: [dataloader: 1, dataloader: 3]
object :cause do
field :id, :id
field :artists, list_of(:artist), resolve: dataloader(Artists)
end
def dataloader do
alias MyApp.{Artists, Causes}
loader = Dataloader.new
|> Dataloader.add_source(Causes, Causes.datasource())
|> Dataloader.add_source(Artists, Artists.datasource())
end
def context(ctx) do
Map.put(ctx, :loader, dataloader())
end
def plugins do
[Absinthe.Middleware.Dataloader] ++ Absinthe.Plugin.defaults()
end
end
From my understanding, with Absinthe Dataloader, the dataloader/1 is what I need to have the list of Artists loaded. However, I am not able to query for the artists from within a cause getting the error artists: #Ecto.Association.NotLoaded<association :artists is not loaded> when I run the query below in graphiql
query{
causes{
id
artists {
id
}
}
}
Is there any little piece that I am missing on working with many to many relationships?
==========
Update
I updated my list_causes function as below
def list_causes do
Repo.all(MyApp.Causes.Cause)
end
to
def list_causes do
Repo.all(from c in Cause,
left_join: ac in "artists_causes", on: c.id == ac.cause_id,
left_join: a in Artist, on: a.id == ac.artist_id,
preload: [:artists]
)
end
and , I am now getting the error FunctionClauseError at POST /graphiql\n\nException:\n\n ** (FunctionClauseError) no function clause matching in anonymous fn/3 in Absinthe.Resolution.Helpers.dataloader/1 which maybe pointing towards with the Absinthe.Resolution.Helpers.dataloader/1 method. I have the helpers imported Is there something else I could be missing?

Answer 1

I think you must preload relation with artists manualy from Ecto, before passing it to Absinthe.
For example, fetch causes like:
from(c in Cause,
preload: [:artists],
select: c
)
|> Repo.all()
ADDITIONAL
My way of resolving Absinthe query.
In query object I pass resolver module function reference.
resolve(&App.Resolver.get_all_causes/2)
And with resolver function I return dataset
def get_all_causes(_params, _info) do
{:ok,
from(c in Cause,
preload: [:artists],
select: c
)
|> Repo.all()}
end