[1.10.x] Fixed #26983 -- Fixed isnull filtering on ForeignKey with to_field

Thanks weidwonder for the report.
Backport of 272eccf7ff0ced609e5a5e3bb6b4a40e773e0e66 from master.

1 files changed, 1 insertions, 1 deletions

diff --git a/django/db/models/fields/related_lookups.py b/django/db/models/fields/related_lookups.py
index 5454031c2a..de2564fb0d 100644
--- a/django/db/models/fields/related_lookups.py
+++ b/django/db/models/fields/related_lookups.py
@@ -93,7 +93,7 @@ class RelatedLookupMixin(object):
             # ForeignKey to IntegerField given value 'abc'. The ForeignKey itself
             # doesn't have validation for non-integers, so we must run validation
             # using the target field.
-            if hasattr(self.lhs.output_field, 'get_path_info'):
+            if self.prepare_rhs and hasattr(self.lhs.output_field, 'get_path_info'):
                 # Get the target field. We can safely assume there is only one
                 # as we don't get to the direct value branch otherwise.
                 target_field = self.lhs.output_field.get_path_info()[-1].target_fields[-1]