Fixed #12663 -- Formalized the Model._meta API for retrieving fields.

Thanks to Russell Keith-Magee for mentoring this Google Summer of
Code 2014 project and everyone else who helped with the patch!

tests/model_fields/models.py

@@ -8,6 +8,11 @@ except ImportError:
     Image = None
 
 from django.core.files.storage import FileSystemStorage
+from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation
+from django.contrib.contenttypes.models import ContentType
+from django.db.models.fields.related import (
+    ForeignObject, ForeignKey, ManyToManyField, OneToOneField,
+)
 from django.db import models
 from django.db.models.fields.files import ImageFieldFile, ImageField
 from django.utils import six
@@ -295,6 +300,52 @@ if Image:
                                   height_field='headshot_height',
                                   width_field='headshot_width')
 
+
+class AllFieldsModel(models.Model):
+    big_integer = models.BigIntegerField()
+    binary = models.BinaryField()
+    boolean = models.BooleanField(default=False)
+    char = models.CharField(max_length=10)
+    csv = models.CommaSeparatedIntegerField(max_length=10)
+    date = models.DateField()
+    datetime = models.DateTimeField()
+    decimal = models.DecimalField(decimal_places=2, max_digits=2)
+    duration = models.DurationField()
+    email = models.EmailField()
+    file_path = models.FilePathField()
+    floatf = models.FloatField()
+    integer = models.IntegerField()
+    ip_address = models.IPAddressField()
+    generic_ip = models.GenericIPAddressField()
+    null_boolean = models.NullBooleanField()
+    positive_integer = models.PositiveIntegerField()
+    positive_small_integer = models.PositiveSmallIntegerField()
+    slug = models.SlugField()
+    small_integer = models.SmallIntegerField()
+    text = models.TextField()
+    time = models.TimeField()
+    url = models.URLField()
+    uuid = models.UUIDField()
+
+    fo = ForeignObject(
+        'self',
+        from_fields=['abstract_non_concrete_id'],
+        to_fields=['id'],
+        related_name='reverse'
+    )
+    fk = ForeignKey(
+        'self',
+        related_name='reverse2'
+    )
+    m2m = ManyToManyField('self')
+    oto = OneToOneField('self')
+
+    object_id = models.PositiveIntegerField()
+    content_type = models.ForeignKey(ContentType)
+    gfk = GenericForeignKey()
+    gr = GenericRelation(DataModel)
+
+
 ###############################################################################