Fixed #20430 - Enable iterable of iterables for model choices

Allows for any iterable, not just lists or tuples, to be used as
the inner item for a list of choices in a model.

tests/invalid_models/invalid_models/models.py

@@ -375,8 +375,8 @@ invalid_models.fielderrors: "decimalfield3": DecimalFields require a "max_digits
 invalid_models.fielderrors: "decimalfield4": DecimalFields require a "max_digits" attribute value that is greater than or equal to the value of the "decimal_places" attribute.
 invalid_models.fielderrors: "filefield": FileFields require an "upload_to" attribute.
 invalid_models.fielderrors: "choices": "choices" should be iterable (e.g., a tuple or list).
-invalid_models.fielderrors: "choices2": "choices" should be a sequence of two-tuples.
-invalid_models.fielderrors: "choices2": "choices" should be a sequence of two-tuples.
+invalid_models.fielderrors: "choices2": "choices" should be a sequence of two-item iterables (e.g. list of 2 item tuples).
+invalid_models.fielderrors: "choices2": "choices" should be a sequence of two-item iterables (e.g. list of 2 item tuples).
 invalid_models.fielderrors: "index": "db_index" should be either None, True or False.
 invalid_models.fielderrors: "field_": Field names cannot end with underscores, because this would lead to ambiguous queryset filters.
 invalid_models.fielderrors: "nullbool": BooleanFields do not accept null values. Use a NullBooleanField instead.

tests/model_validation/models.py

@@ -0,0 +1,27 @@
+from django.db import models
+
+
+class ThingItem(object):
+
+    def __init__(self, value, display):
+        self.value = value
+        self.display = display
+
+    def __iter__(self):
+        return (x for x in [self.value, self.display])
+
+    def __len__(self):
+        return 2
+
+
+class Things(object):
+
+    def __iter__(self):
+        return (x for x in [ThingItem(1, 2), ThingItem(3, 4)])
+
+
+class ThingWithIterableChoices(models.Model):
+
+    # Testing choices= Iterable of Iterables
+    #   See: https://code.djangoproject.com/ticket/20430
+    thing = models.CharField(max_length=100, blank=True, choices=Things())

tests/model_validation/tests.py

@@ -0,0 +1,14 @@
+import io
+
+from django.core import management
+from django.test import TestCase
+
+
+class ModelValidationTest(TestCase):
+
+    def test_models_validate(self):
+        # All our models should validate properly
+        # Validation Tests:
+        #   * choices= Iterable of Iterables
+        #       See: https://code.djangoproject.com/ticket/20430
+        management.call_command("validate", stdout=io.BytesIO())