Fixed #31486 -- Deprecated passing unsaved objects to related filters.

Co-Authored-By: Hasan Ramezani <hasan.r67@gmail.com>

django/db/models/fields/related_lookups.py

@@ -1,3 +1,5 @@
+import warnings
+
 from django.db.models.lookups import (
     Exact,
     GreaterThan,
@@ -7,6 +9,7 @@ from django.db.models.lookups import (
     LessThan,
     LessThanOrEqual,
 )
+from django.utils.deprecation import RemovedInDjango50Warning
 
 
 class MultiColSource:
@@ -40,6 +43,15 @@ def get_normalized_value(value, lhs):
     from django.db.models import Model
 
     if isinstance(value, Model):
+        if value.pk is None:
+            # When the deprecation ends, replace with:
+            # raise ValueError(
+            #     "Model instances passed to related filters must be saved."
+            # )
+            warnings.warn(
+                "Passing unsaved model instances to related filters is deprecated.",
+                RemovedInDjango50Warning,
+            )
         value_list = []
         sources = lhs.output_field.path_infos[-1].target_fields
         for source in sources: