Define internal types explicitly for related fields

[Andriy Sokolovskiy]

For related fields internal type is not defined explicitly.
For example, if I want to inherit these fields to add some minor changes, I will need to define internal type manually, because some of backend code is related on internal type of the field.

Old pull request - ​https://github.com/django/django/pull/4002

We discussed here that is a good idea to have this feature in 1.9

[Andriy Sokolovskiy]

​https://github.com/django/django/pull/4296

[Tim Graham]

I didn't understand the need to add this. Probably some tests are needed to show the use cases.

[Andriy Sokolovskiy]

@timgraham, well, it was more suitable for 1.7.x, where we had many places with internal type checks.
At a quick glance in master I found this ​https://github.com/django/django/blob/a52cd407b86a51e1badf6771e590361e24fd7155/django/db/backends/mysql/schema.py#L61
There may be also places, where are similar checks.
The main goal to give ability for users to not set internal types manually if they need to override one of related fields and give some minor changes.

[Tim Graham]

I guess this is consistent with the other built-in fields. Could you add regression tests?

[Akash Kumar Sen]

​PR

[Mariusz Felisiak]

I don't see a real use case for adding this. Moreover, others do not seem too convinced either and it can cause issue for custom subclasses of related fields. As far as I'm aware, it's not worth changing this long-standing behavior and potential hiccups.