Document how to write a custom lookup where the rhs comes before the lhs

[JJ Gutierrez]

The page for creating custom field lookups is incomplete. I am referring to the doc at ​https://docs.djangoproject.com/en/2.0/howto/custom-lookups/. The documentation is unclear about how to use the left-hand side value (lhs) and right-hand side value (rhs) properly. For example, lets say you have a lookup like

id__custom='abc123'

The lhs is the ID field and the rhs is the 'abc123'. I want to create a query like

WHERE 'abc123' LIKE id||%

However the documentation is unclear about how to reference the lhs in the as_sql method. Users would like to know how to reference the rhs BEFORE the LIKE and the lhs AFTER the LIKE in the sql query above. The documentation only shows examples where the lhs comes first and the rhs last. This would help users create a custom lookup with the sql above.

[Tim Graham]

Maybe looking at ​the built in lookups would offer a hint.

[Mariusz Felisiak]

However the documentation is unclear about how to reference the lhs in the as_sql method. Users would like to know how to reference the rhs BEFORE the LIKE and the lhs AFTER the LIKE in the sql query above. The documentation only shows examples where the lhs comes first and the rhs last. This would help users create a custom lookup with the sql above.

IMO ​A lookup example makes it really clear, I don't see how we could make it clearer. You need to swap lhs and rhs, e.g.:

class Custom(Lookup):
    lookup_name = 'custom'

    def as_sql(self, compiler, connection):
        lhs, lhs_params = self.process_lhs(compiler, connection)
        rhs, rhs_params = self.process_rhs(compiler, connection)
        params = rhs_params + lhs_params
        return '%s LIKE %s||%%' % (rhs, lhs), params

Closing as invalid.