Remove rev_join_map from sql/query.py

[Anssi Kääriäinen]

The sql.query.QuerySet.rev_join_map is used currently in two places of the query.py:

1. In query.combine(). The information used there is available directly from alias_map
2. In relabel_aliases as a fast path to join_map. However, one can just iterate through the join_map and relabel the aliases directly.

It is hard to see if the usage in 1. is actually currently correct or not: when combining queries the joins are re-added using _table_ name for the left hand side of the join (the thing we are joining into). I am pretty sure the correct way would be to do that using the existing lhs _alias_ so that the joins are added to the same previous join as in the original query.

While the cleanup doesn't promise much advancement, it would give a (really) small speedup to query cloning. Just getting rid of one variable is reason enough for removal I guess.

[Anssi Kääriäinen]

The attached patch does the mentioned removal. I am actually pretty sure the rev_join_map usage in query.combine is broken: the intention is to use the left hand side table's alias, but rev_join_map has the table instead of the alias. All tests passed on SQLite3.

[Alex Gaynor]

Looks pretty reasonable to me, check in at your discretion!

[Anssi Kääriäinen]

In [17878]:

Fixed #18014 -- Removed rev_join_map from sql/query.py.