[5.5] Allow setting custom morphTo ownerKey

[v0ctor]

As proposed at laravel/ideas#587, this PR adds the ability to set a custom ownerKey when defining a morphTo polymorphic relationship.

Method definition is equivalent to morphOne and morphMany, among others, so it brings consistency to relationship definitions while providing the freedom to use them with data models that can not be easily modified. It is fully backwards compatible with Laravel 5.5 LTS.

Please tell me what you think about this PR. If you know any way to do it better or you think that it is unsuitable, feel free to give your opinion. Thanks!

[taylorotwell]

It's not fully backwards compatible because it changes function signatures.

[v0ctor]

@taylorotwell I think I do not understand the problem. The new parameter has a default value, so the new function signature of morphTo is compatible with the old one. How does this affect backward compatibility?

Anyway, what would be the correct way to implement it? Or should I have done the pull request on the master branch?

[v0ctor]

I have resubmitted this PR to master (#21310). I hope it's the right way.

[paulofreitas]

@victordzmr Every function signature change is a potential breaking change. When changing a class/trait that is intended to be extended by the user it's always a BC break because people often overload those methods to change the framework behavior. Contracts on other hand should never be changed as their changes would always break things.

Some BC breaks may impact the user base badly and others may went unnoticed. In those situations submitting the change to a future release is always the safest choice. 😉

[v0ctor]

@paulofreitas I had not thought of that possibility. Thank you very much for the explanation! 🙂