Support limit queries with Dataloader.Ecto

[mbuhot]

This PR is a proof of concept for implementing #51

With Ecto now supporting parent_as, lateral joins are now possible without fragments.

The implementation loads the inputs into a CTE to drive a lateral join query.
Unfortunately it is using the postgres-specific unnest function, which also requires explicitly casting the inputs to an array type. Any suggestions on making it more portable/general?

Edit: Found a way to make it work without fragments or casting.

Update: I've added support for preloading associations using lateral joins too, this makes it very convenient to use from absinthe resolvers using the dataloader/1 resolution helper.

[benwilson512]

This is amazing, fantastic work. Gonna mess with it some. Can you include a test case or two?

[mbuhot]

Thanks @benwilson512!

I've been prototyping with this app based on the Chinook dataset: https://github.com/mbuhot/chinook/blob/master/test/chinook_web/graphql/playlist_test.exs

It works wonderfully to declare nested relay connections with a 1 line resolution helper :)

  node object(:playlist) do
    field :name, non_null(:string)

    connection field :tracks, node_type: :track do
      arg :by, :track_sort_order, default_value: :track_id
      arg :filter, :track_filter, default_value: %{}
      resolve Relay.connection_dataloader(Chinook.Track.Loader) # resolves through a many-to-many relationship
    end
  end

I'll add some tests that exercise some of the variations on HasMany, HasThrough, ManyToMany.

[benwilson512]

Incredible! I'll pull this down today and mess with it but this is looking basically perfect so far.

[mbuhot]

@benwilson512 I've added some more code and tests to cover some edge cases I ran into.

Let me know if the code can be better organised. I'm starting to wonder if the preload_lateral machinery would be better in Ecto itself rather than in Dataloader.Ecto 🤔

[benwilson512]

I'm starting to wonder if the preload_lateral machinery would be better in Ecto itself rather than in Dataloader.Ecto

Agreed. Perhaps see if there is interest for a PR on ecto to add that. If not we can go ahead and just add it here.

[benwilson512]

@mbuhot one thing I'm noticing is that Dataloader still specifies its ecto dependency as >= 0.0.0. Can you make that more precise to whatever version adds the preload support you require?

[mbuhot]

Will do 👍 3.4.3 provides parent_as which is the key to it.

[benwilson512]

Thank you!