accept an optional ctx parameter on model methods

[gracenoah]

This PR allows you to take a context on model methods. This makes it much easier to transition from graph-gophers/graphql-go to this library. By configuring all of your graph-gophers resolvers as "models" in gqlgen, you can reuse existing code and migrate gradually instead of having to rewrite all resolvers in order to migrate.

Testing and docs coming up after I get some initial feedback on the concept and implementation.

Edit: I ran into one more thing that prevents me from being able to transition gradually. Models can't be returned from resolvers as pointers unless they are optional. All of my graph-gophers resolvers are pointers. I'm considering a temporary hack to make all models passed as pointers or converting all graph-gophers resolvers to non-pointers (there was no real reason for them to be pointers). I'd love some tips if anyone has done this before.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[vektah]

Overall I think this makes sense, one thing you might run into though is gqlgen doesn't run bound model methods in parallel, its assumed they don't hit the database. Its an optimization mostly to avoid bound fields from spawning goroutines. Maybe context is an indicator that it should be run in parallel?

We are thinking about making all non primative return types pointers, which may help the second issue. See #375

[gracenoah]

Ah, thanks for the tip about not running in parallel. That might make it infeasible to reuse the resolvers without rewriting them. I'll try to address that in this PR as well. Parallelizing if there's a context makes sense to me.

P.S. Added my +1 on the pointers issue.

[gracenoah]

I don't see any existing tests for model methods. Any suggestions on where I should add the tests?

[vektah]

You can create a model in codegen/testserver and wire it into that config, then add tests to generated_test.go (or create a new file like modelctx_test.go) in that directory.

[gracenoah]

Tests added. Please review.

[vektah]

You need to run a go generate ./... and commit the changes. Seeing how the generated code has changed makes reviewing easier (even if it artificially inflates line counts)

[gracenoah]

Sorry, didn't realize that resolver.go is generated. Updated.

[gracenoah]

Sorry about the testing screwups. It's green now.

[vektah]

Awesome, thanks for the PR.

Looks like we could use a guide for migrating from gophers, If anyone has a large codebase they have successfully migrated it would be great to get their notes (maybe in the recipes section?).