Implement new config to specify return type of custom relations

[Jefemy]

Summary

Custom Relation types can be added using the config additional_relation_types already, but in the backend the return types for those relations is determined by using a simple check determining if the class name has Many in it to decide the return type of the relation.

This works for the built-in Laravel relations where they are named properly but packages like https://github.com/staudenmeir/eloquent-json-relations use a name such as belongsToJson to return a many relationship. This change allows a config option to specify the actual return type of the relation such as many or morphTo

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist

• Existing tests have been adapted and/or new tests have been added
• Add a CHANGELOG.md entry
• Update the README.md
• Code style has been fixed via composer fix-style

[mfn]

LGTM, left a minor feedback and can you please also cover not just the 'many' test but also 'morphTo'? (or did I miss it?)

[mfn]

Oh, and please also add it to the README; otherwise it's hard to discover for others.

[Jefemy]

Oh, and please also add it to the README; otherwise it's hard to discover for others.

I added a test for the morphTo case. I'm not sure where to put documentation on this feature. The config option additional_relation_types which is already required for this to work isn't documented anywhere other than the config file. Should I write up something for that option too?

[mfn]

Should I write up something for that option too?

If you can spare the time, this would be terrific! I believe that mentioning things in the README (aka our primary docs) definitely makes things easier to figure out for other developers.

PS: you can ignore the failed tests, I've a separate PR for fixing this in #1351

[mfn]

Yissir, LGTM, thanks for your work!

ping @barryvdh