[1.5.x] Fixed #19445 -- Skip admin fieldsets validation when the ModelAdmin.get_form() method is overridden.

Backport of 0694d2196f

tests/regressiontests/admin_validation/tests.py

@@ -20,6 +20,18 @@ class InvalidFields(admin.ModelAdmin):
     form = SongForm
     fields = ['spam']
 
+class ValidFormFieldsets(admin.ModelAdmin):
+    def get_form(self, request, obj=None, **kwargs):
+        class ExtraFieldForm(SongForm):
+            name = forms.CharField(max_length=50)
+        return ExtraFieldForm
+
+    fieldsets = (
+        (None, {
+            'fields': ('name',),
+        }),
+    )
+
 class ValidationTestCase(TestCase):
 
     def test_readonly_and_editable(self):
@@ -42,6 +54,14 @@ class ValidationTestCase(TestCase):
             validate,
             InvalidFields, Song)
 
+    def test_custom_get_form_with_fieldsets(self):
+        """
+        Ensure that the fieldsets validation is skipped when the ModelAdmin.get_form() method
+        is overridden.
+        Refs #19445.
+        """
+        validate(ValidFormFieldsets, Song)
+
     def test_exclude_values(self):
         """
         Tests for basic validation of 'exclude' option values (#12689)