Model._meta proxy_for_model set up incorrectly

[Anssi Kääriäinen]

There are some places in the code which assume that proxy_for_model creates a proxy chain. That is, if you have models:

A
Proxy(A)
ProxyProxy(Proxy)

the expected result is that ProxyProxy._meta.proxy_for_model == Proxy, but this isn't true. One place which makes this proxy-chain mistake is the very place that sets up the proxy_for_model so this seems like a clear bug.

In the attached patch I added ._meta.concrete_class, which is always the concrete class of the model. This simplifies code in some places. In addition, the proxy_for_model correctly creates a proxy chain now.

I think the proxy_for_model error was responsible for one bug in admin, but I haven't verified that there was a bug (so, no tests). In addition there is one place in sql/query.py which is working now as it was before, but I am not sure if the before behavior was in fact correct. Neither of these should be this ticket's problems.

The bug is present in at least 1.3, maybe even before.

[Carl Meyer]

In [17573]:

Fixed #17678 -- Corrected setup of _meta.proxy_for_model and added _meta.concrete_model. Thanks Anssi Kääriäinen.