feat: add early fk_field configuration validation

django_admin_reversefields/mixins.py

@@ -50,6 +50,7 @@
 
 from django import forms
 from django.contrib.admin.widgets import FilteredSelectMultiple
+from django.core.exceptions import FieldDoesNotExist, ImproperlyConfigured
 from django.db import models, transaction
 from django.http import HttpRequest
 
@@ -475,6 +476,41 @@ def get_reverse_relations(self) -> dict[str, ReverseRelationConfig]:
         """
         return self.reverse_relations
 
+    def _validate_reverse_relation_configs(self) -> None:
+        """Validate reverse relation configuration before form construction.
+
+        Raises:
+            ImproperlyConfigured: If a reverse relation has an invalid ``fk_field``
+                (missing, wrong type, or targeting a different parent model).
+        """
+        for field_name, config in self.get_reverse_relations().items():
+            try:
+                fk = config.model._meta.get_field(config.fk_field)
+            except FieldDoesNotExist as exc:
+                raise ImproperlyConfigured(
+                    f"Invalid reverse_relations['{field_name}']: fk_field "
+                    f"'{config.fk_field}' does not exist on model '{config.model._meta.label}'."
+                ) from exc
+
+            if not isinstance(fk, (models.ForeignKey, models.OneToOneField)):
+                raise ImproperlyConfigured(
+                    f"Invalid reverse_relations['{field_name}']: fk_field "
+                    f"'{config.fk_field}' on model '{config.model._meta.label}' must be "
+                    "a ForeignKey or OneToOneField."
+                )
+
+            target_model = fk.remote_field.model
+            admin_model = self.model
+            if not (
+                issubclass(admin_model, target_model) or issubclass(target_model, admin_model)
+            ):
+                raise ImproperlyConfigured(
+                    f"Invalid reverse_relations['{field_name}']: fk_field "
+                    f"'{config.fk_field}' on model '{config.model._meta.label}' points to "
+                    f"'{target_model._meta.label}', but this admin manages "
+                    f"'{admin_model._meta.label}'."
+                )
+
     def get_fields(self, request, obj=None):  # type: ignore[override]
         """Ensure virtual reverse field names are part of the rendered fields.
 
@@ -514,6 +550,8 @@ def get_form(self, request: HttpRequest, obj=None, **kwargs):
             the configured reverse relation fields.
         """
         relations = dict(self.get_reverse_relations())
+        if relations:
+            self._validate_reverse_relation_configs()
         if relations:
             provided_fields = kwargs.get("fields")
             if provided_fields:

tests/test_edge_cases.py

@@ -2,19 +2,71 @@
 
 # Test imports
 from django.contrib import admin
+from django.core.exceptions import ImproperlyConfigured
 from django.db import transaction
 
 from django_admin_reversefields.mixins import (
     ReverseRelationAdminMixin,
     ReverseRelationConfig,
 )
-from tests.models import Company, CompanySettings, Department, Project
+from tests.models import Company, CompanySettings, Department, Employee, Project
 from tests.shared_test_base import BaseAdminMixinTestCase
 
 
 class EdgeCasesTests(BaseAdminMixinTestCase):
     """Test suite for edge cases and non-parameterizable scenarios."""
 
+    def test_invalid_fk_field_name_fails_early(self):
+        """Misconfigured fk_field names should fail during form construction."""
+
+        class TestAdmin(ReverseRelationAdminMixin, admin.ModelAdmin):
+            reverse_relations = {
+                "bad_binding": ReverseRelationConfig(
+                    model=Department,
+                    fk_field="does_not_exist",
+                ),
+            }
+
+        admin_inst = TestAdmin(Company, self.site)
+        request = self.factory.get("/")
+
+        with self.assertRaisesMessage(ImproperlyConfigured, "does not exist on model"):
+            admin_inst.get_form(request, self.company)
+
+    def test_non_relational_fk_field_fails_early(self):
+        """fk_field must reference a ForeignKey or OneToOneField."""
+
+        class TestAdmin(ReverseRelationAdminMixin, admin.ModelAdmin):
+            reverse_relations = {
+                "bad_binding": ReverseRelationConfig(
+                    model=Department,
+                    fk_field="name",  # CharField, not a relation
+                ),
+            }
+
+        admin_inst = TestAdmin(Company, self.site)
+        request = self.factory.get("/")
+
+        with self.assertRaisesMessage(ImproperlyConfigured, "must be a ForeignKey or OneToOneField"):
+            admin_inst.get_form(request, self.company)
+
+    def test_fk_field_target_model_mismatch_fails_early(self):
+        """fk_field target model must match the admin's parent model."""
+
+        class TestAdmin(ReverseRelationAdminMixin, admin.ModelAdmin):
+            reverse_relations = {
+                "bad_binding": ReverseRelationConfig(
+                    model=Employee,
+                    fk_field="department",  # Points to Department, not Company
+                ),
+            }
+
+        admin_inst = TestAdmin(Company, self.site)
+        request = self.factory.get("/")
+
+        with self.assertRaisesMessage(ImproperlyConfigured, "but this admin manages"):
+            admin_inst.get_form(request, self.company)
+
     def test_large_dataset_performance(self):
         """Test base operations with large datasets."""
         # Create a large dataset