Refactor sql.query.select and select_fields

[Anssi Kääriäinen]

I introduced some major gis regressions in commit [f64a5ef404cb6fd28e008a01039a3beea2fa8e1b] - a fix for #19102. The reason for the regressions was that Query.select and select_fields must be in sync, but this applies only to gis backend. The commit did:

qs.select = [something]

while it should have done:

qs.select, qs.select_fields = [something], [None]

The qs.select and select_fields must always be edited in sync for gis to work. There are not even comments explaining how select and select_fields work currently.

I have implemented ​a refactor for this which removes select_fields and collapses it into qs.select. This way it will be impossible to do the same mistake I did in future.

It seems similar refactoring should be done for related_select_cols/related_select_fields.

[Anssi Kääriäinen]

I have pushed a commit to the ​branch to do similar collapsing for related_select_cols and related_select_fields.

[Anssi Kääriäinen]

The branch has been rebased, and is now RFC.

[Alex Gaynor]

django/contrib/gis/db/models/sql/compiler.py: line 88, remove the zip

otherwise LGTM, please merge

[Anssi Kääriäinen <akaariai@…>]

In 11699ac4b5f98ec11dba02b356a8fd4ab6b4b889:

Fixed #19190 -- Refactored Query select clause attributes

The Query.select and Query.select_fields were collapsed into one list
because the attributes had to be always in sync. Now that they are in
one attribute it is impossible to edit them out of sync.

Similar collapse was done for Query.related_select_cols and
Query.related_select_fields.