Add union support

[DmitryTsepelev]

No description provided.

[jeromedalbert]

I guess that you didn't manage to make this workaround PR work, or the specs never passed?

We are using a few unions in our graphql app, so I am getting those Failed to look ahead the field errors. Since the impacted cache_fragment: fields are sometimes used in vastly different queries and selections, I don't think I can safely pass a custom (mostly static) query_cache_key as a workaround, since the respective fragments could look wildly different and this could result in serving wrong cached fragments. Unless there is some way to fingerprint a graphql query selection without using lookaheads, but I don't know of one.

I tried using rmosolgo/graphql-ruby#3007 's branch in my app and still got the Failed to look ahead the field error. I am guessing that GraphQL::FragmentCache would need to somehow adapt to rmosolgo/graphql-ruby#3007 if it gets merged one day.

[DmitryTsepelev]

The problem is that lookahead does not support unions, and we build a cache key based on it. I feel like it's easier to find a different reliable way to build a key without the lookahead rather than try to fix it's behaviour.

[hendie]

Any progress on this PR @DmitryTsepelev ? We also have quite a few instances of unions in our data model. I would love to use your cache offering, but that's kinda limiting us from further evaluation at this time.

[jeromedalbert]

FYI I worked around the issue in my app by not using union types, and using interfaces instead. It seemed to mostly work.

Unions and interfaces in GraphQL are very similar, so migrating required minimal effort. No changes were needed for the frontend / API consumer, since the ... on syntax in queries remained the same.