id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
21863	Consider adding get_transform() to supplement get_lookup()	Anssi Kääriäinen	nobody	"Currently the API for get_lookup() is get_lookup(lookup_name). However, there are situations where a field will need more information to decide if it needs to return a lookup or a transform. Consider HStoreField. A candidate API for HStoreField is that:
  - `hstore_field__exact=val` => exact lookup against the hstore_field
  - `hstore_field=val` => also exact lookup against the hstore_field
  - `hstore_field__exact__exact=val` => get key exact from hstore, do an exact lookup against that
  - `hstore_field__nonlookup=val` => get key nonlookup from hstore, do an exact lookup against that


This is similar to how `fk__exact` and `fk__exact__exact` work currently if the model pointed by fk has field named exact.

In addition, we want to support transforms in .values() etc calls in the future. So `hstore_field__exact` should always resolve to transform if used in values().

Currently this is impossible to do. The get_lookup() method doesn't know if there are still further lookups in the path. It knows just the current lookup. If we are resolving path `exact__exact` get_lookup() should return transform for the first `exact`, lookup for the second `exact`. If we are resolving just `exact`, then lookup should be returned for the first 'exact'. When resolving `exact` for `.values('hstorefield__exact')` call in later releases, the `exact` should resolve to transform.

If we add get_transform() method we could change the lookup resolution code in the ORM to do:
  - for non-final lookups: always use get_transform()
  - for final lookup in filter() calls: try get_lookup(lookup_name). If that doesn't return anything, use get_transform(lookup_name) + get_lookup('exact').
  - for any lookup in .values() etc calls: always use get_transform()

The above examples would be resolved as follows:
  - `hstorefield__exact=val`: get_lookup('exact') -> match, so use exact lookup against hstorefield
  - `hstorefield=val`: get_lookup('exact') -> same as above
  - `hstorefield__exact__exact=val`: get_transform('exact'), get_lookup('exact') -> match, so use exact lookup against key exact of hstorefield
  - `hstorefield__nonlookup=val`: get_lookup('nonlookup') -> no match, so try get_transform('nonlookup') + get_lookup('exact') -> exact lookup against key nonlookup

The values() call will use always get_transform() so `.values('hstorefield__exact`)` will work correctly.

There might also be need to add register_transform() method. If we have only register_lookup() but methods get_lookup() and get_transform() it might lead to some confusion.

Adding register_transform() will break the API, but that is hopefully OK during alpha."	New feature	closed	Database layer (models, ORM)	1.7-alpha-1	Normal	fixed			Accepted	1	1	0	0	0	0