Add validation of related fields and lookups in model Meta.ordering

[Shadi Akiki]

When the ordering class member in Meta of a model contains a field from a related model, and that field does not exist, django's makemigrations does not throw an error. However, if it is a direct field member of the same class, makemigrations does throw an error.
Example below tested on Django 2.0.5

from django.db import models

# Create your models here.

class Agreement(models.Model):
    agreement_id = models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)
    #class Meta:
      # generates error in makemigrations
      # app.Agreement: (models.E015) 'ordering' refers to the nonexistent field 'id'.
      # ordering = ['id']

class Order(models.Model):
    agreement = models.ForeignKey(Agreement, models.DO_NOTHING)
    class Meta:
      # does not generate error in makemigrations
      # but does so during runtime
      # e.g. [x for x in Order.objects.all()]
      ordering = ['agreement__id']

[Carlton Gibson]

I'm going to accept this provisionally. There's ​a `FIXME` in `models/base.py` specifically about this:

        # Skip ordering in the format field1__field2 (FIXME: checking
        # this format would be nice, but it's a little fiddly).
        fields = (f for f in fields if LOOKUP_SEP not in f)

Added in ​d818e0c9b2b88276cc499974f9eee893170bf0a8.

Either we should address this, or remove the comment and close as wontfix if "fiddly" turns out to be more effort than it's worth.

A test case and a patch showing what "fiddly" actually entails would be great.

[Windson yang]

I think we can just address this in the document and don't fix it.

[Hasan Ramezani]

​PR

[Hasan Ramezani]

patch updated and new method added.

[Hasan Ramezani]

Any updates? if there is something to change please inform me. I am ready.

[Carlton Gibson]

Left comments on PR: patch would need to handle JSON paths, which should be valid in ordering since #24747. (Similar issue arises in #29622.)

[Hasan Ramezani]

PR updated. JSON paths handled and some tests added for it

[Carlton Gibson]

Patch is looking good. There's just a few minor comments on the PR, but when those are addressed it should be ready to move forwards.

[Hasan Ramezani]

Requested changes are done

[Mariusz Felisiak <felisiak.mariusz@…>]

In 440505cb:

Fixed #29408 -- Added validation of related fields and lookups in model Meta.ordering.

[GitHub <noreply@…>]

In f69c7bbd:

Refs #29408 -- Cosmetic edits for validation of related fields and lookups in model Meta.ordering.

Follow up to 440505cb2cadbe1a5b9fba246bcde6c04f51d07e.