Fixed #24767 -- Added Greatest and Least expressions

[LilyFirefly]

For ticket #24767.

[charettes]

Greatest() should do.

[LilyFirefly]

This is still failing on MySQL. It appears that now the annotated values are not being converted back into the correct python types. Actually, this seems to have been a build fluke.

[LilyFirefly]

What do I still need to do for this? I know documentation is needed. Do I need to add any more tests, for example for other databases?

[jarshwah]

On Mysql, Oracle, and SQLite

[jarshwah]

You still need to implement Least (which is just the reverse of Greatest). I think the duality means both should ship in the same commit.

I'd also like to see some tests that used related fields:

.annotate(eldest_friend=Greatest('friends', 'friends__age'))

The tests should cover non-null and null relations to have the best coverage.

I'd also like to see a test that uses Greatest for updating an existing field:

.update(last_updated=Greatest('comments__datetime', 'last_updated'))

I'm using made up fields above, but there should be models available that you can find a use case for. If not, it's ok to create a new model that fits the need.

Once that's all done (and docs are written etc) it'd be good if you could squash all the commits into one, to make it easier on whoever will merge the PR. But we'll review some more before that's necessary anyway.

[LilyFirefly]

Yeah, I was waiting to do Least until everything had been worked out with Greatest.

[LilyFirefly]

@jarshwah Is this the sort of test you were thinking of?
It currently fails with a FieldError that looks related to #14104.

Traceback (most recent call last):
  File "/home/ian/dev/django/tests/db_functions/tests.py", line 164, in test_greatest_update
    Author.objects.update(age=Greatest('age', 'fans__age'))
  File "/home/ian/dev/django/django/db/models/manager.py", line 125, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/ian/dev/django/django/db/models/query.py", line 619, in update
    rows = query.get_compiler(self.db).execute_sql(CURSOR)
  File "/home/ian/dev/django/django/db/models/sql/compiler.py", line 1065, in execute_sql
    cursor = super(SQLUpdateCompiler, self).execute_sql(result_type)
  File "/home/ian/dev/django/django/db/models/sql/compiler.py", line 832, in execute_sql
    sql, params = self.as_sql()
  File "/home/ian/dev/django/django/db/models/sql/compiler.py", line 1018, in as_sql
    val = val.resolve_expression(self.query, allow_joins=False, for_save=True)
  File "/home/ian/dev/django/django/db/models/expressions.py", line 491, in resolve_expression
    c.source_expressions[pos] = arg.resolve_expression(query, allow_joins, reuse, summarize, for_save)
  File "/home/ian/dev/django/django/db/models/expressions.py", line 448, in resolve_expression
    return query.resolve_ref(self.name, allow_joins, reuse, summarize)
  File "/home/ian/dev/django/django/db/models/sql/query.py", line 1416, in resolve_ref
    raise FieldError("Joined field references are not permitted in this query")
django.core.exceptions.FieldError: Joined field references are not permitted in this query

[jarshwah]

Yes it was Ian, but my example used joined fields for an update which isn't (yet) allowed.

How about something like:

Author.objects.update(alias=Greatest('name', 'goes_by')

Which will also test the handling of varchars in a Greatest.

[mjtamlyn]

buildbot, test on oracle.

[mjtamlyn]

Looks like your coalesce workaround fails on MySQL. Good news is Oracle is fine.

[LilyFirefly]

See #24925.

[LilyFirefly]

It looks like the buildbot didn't pick up my most recent change (adding test_greatest_coalesce_workaround_mysql and test_least_coalesce_workaround_mysql).

[mjtamlyn]

buildbot, test this please

[mjtamlyn]

Merged in 4ab53a5