Prefetech Object - to_attr has ambiguous behavior when using a related lookup

[Tal Perry]

Code Example

from django.db.models import Prefetch

p = Prefetch("a_set", to_attr="my_a")
assert p.prefetch_to == "my_a"

p = Prefetch("b__a_set", to_attr="my_a")
assert p.prefetch_to == "my_a", p.prefetch_to

That last assertion fails because p.prefetch_to == "b__my_a"

Description and use case
We have a models X, Y and Z
Z has a foreign key to Y and Y has a foreign key to Z
We'd like to fetch X with all of it's Z's using prefetch related and to be able to set the attribute to "z"

But as above, this doesn't seem to work.

The ​relevant Django code seems to ignore this use case (or discourage it?) and instead puts to "z" attribute on each "Y"

            self.prefetch_to = LOOKUP_SEP.join(lookup.split(LOOKUP_SEP)[:-1] + [to_attr])

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I assume "fixing this" is a breaking change, but could we do an explicit parameter for prefetch_to ?At the moment we're setting it manually after creating the prefetch object

[Mariusz Felisiak]

Thanks for this ticket, however IMO this is an expected behavior. In the second example a_set is a field lookup on b so we can change its name to my_a (in b) but we cannot assign it directly to the main object, it will be misleading.