🐛 RelationConnection loads the same relation twice under AsyncDataloader (2.6.7)

[benschwarz]

Given a query like:

{ posts(first: 20) { edges { node { id } } pageInfo { endCursor } } }

Both fields call load_nodes. 2.6.7 runs them concurrently, earlier releases didn't.

@nodes ||= limited_nodes.to_a   # relation_connection.rb:224

Fiber one waits on the DB before setting @nodes. Fiber two sees empty and loads the same relation again. Whichever finishes first clears ActiveRecord's load state from under the other.

This results in:

NoMethodError: undefined method 'instantiate' for nil, plus a duplicate query on every paginated connection.

[rmosolgo]

Hey, thanks for reporting this and sorry for the trouble 😖

I'm hesitant to address this with dataloader.yield because I expect this to all happen inside GraphQL-Ruby's execution steps -- without handing off to Dataloader to fetch results from Dataloader sources.

Do you have a guess why the Fiber is yielding here? Is this a custom connection implementation?