Get rid of field.get_db_prep_lookup()

[Anssi Kääriäinen]

The Field.get_db_prep_lookup() method is used to do preparation of a plain Python value for given lookup. Looking at the coding of ​get_db_prep_lookup() the get_db_prep_lookup() is doing work that belongs into the lookup itself.

The same goes also for get_prep_lookup(). It seems that get_prep_lookup() is either doing something that should actually be done in get_prep_value(), or something that belongs into lookups.

If a custom field needs to do different kind of preparation for some lookups (something that can't be done in get_[db_]prep_value(), it can always provide a lookup subclass that does the right thing for that field type. An example is ​IntegerField and 'lt' and 'gte' lookups. Providing custom subclasses might be laborious to do if there are a lot of different lookup types that need custom preparation. For that reason a hook for custom fields could still be useful.

Even if we want to leave get_db_prep_lookup and get_prep_lookup hooks for custom fields, the base coding belongs into the lookups themselves. Currently the fields are doing work that clearly belongs to the lookup itself.

[Tim Graham]

As suggested in the ticket description, I did the small step of moving IntegerField.get_prep_lookup() to lookup subclasses: ​PR.