[1.7.x] Fixed #22537 -- Add tests and improved docs for field subclass with choices.

Backport of 7fd1b35ed7 from master

docs/ref/models/fields.txt

@@ -83,8 +83,8 @@ two items (e.g. ``[(A, B), (A, B) ...]``) to use as choices for this field. If
 this is given, the default form widget will be a select box with these choices
 instead of the standard text field.
 
-The first element in each tuple is the actual value to be stored, and the
-second element is the human-readable name. For example::
+The first element in each tuple is the actual value to be set on the model,
+and the second element is the human-readable name. For example::
 
     YEAR_IN_SCHOOL_CHOICES = (
         ('FR', 'Freshman'),

tests/field_subclassing/fields.py

@@ -20,6 +20,11 @@ class Small(object):
     def __str__(self):
         return '%s%s' % (force_text(self.first), force_text(self.second))
 
+    def __eq__(self, other):
+        if isinstance(other, self.__class__):
+            return self.first == other.first and self.second == other.second
+        return False
+
 
 class SmallField(six.with_metaclass(models.SubfieldBase, models.Field)):
     """

tests/field_subclassing/models.py

@@ -5,7 +5,7 @@ Tests for field subclassing.
 from django.db import models
 from django.utils.encoding import force_text
 
-from .fields import SmallField, SmallerField, JSONField
+from .fields import Small, SmallField, SmallerField, JSONField
 from django.utils.encoding import python_2_unicode_compatible
 
 
@@ -22,5 +22,15 @@ class OtherModel(models.Model):
     data = SmallerField()
 
 
+class ChoicesModel(models.Model):
+    SMALL_AB = Small('a', 'b')
+    SMALL_CD = Small('c', 'd')
+    SMALL_CHOICES = (
+        (SMALL_AB, str(SMALL_AB)),
+        (SMALL_CD, str(SMALL_CD)),
+    )
+    data = SmallField('small field', choices=SMALL_CHOICES)
+
+
 class DataModel(models.Model):
     data = JSONField()

tests/field_subclassing/tests.py

@@ -2,12 +2,12 @@ from __future__ import unicode_literals
 
 import inspect
 
-from django.core import serializers
+from django.core import exceptions, serializers
 from django.db import connection
 from django.test import TestCase
 
 from .fields import Small, CustomTypedField
-from .models import DataModel, MyModel, OtherModel
+from .models import ChoicesModel, DataModel, MyModel, OtherModel
 
 
 class CustomField(TestCase):
@@ -106,6 +106,16 @@ class CustomField(TestCase):
         self.assertIn('__module__', data)
         self.assertEqual(data['__module__'], 'field_subclassing.models')
 
+    def test_validation_of_choices_for_custom_field(self):
+        # a valid choice
+        o = ChoicesModel.objects.create(data=Small('a', 'b'))
+        o.full_clean()
+
+        # an invalid choice
+        o = ChoicesModel.objects.create(data=Small('d', 'e'))
+        with self.assertRaises(exceptions.ValidationError):
+            o.full_clean()
+
 
 class TestDbType(TestCase):