Fixed #13774 -- Added models.Field.rel_db_type().

django/db/models/fields/__init__.py

@@ -626,6 +626,14 @@ class Field(RegisterLookupMixin):
         except KeyError:
             return None
 
+    def rel_db_type(self, connection):
+        """
+        Return the data type that a related field pointing to this field should
+        use. For example, this method is called by ForeignKey and OneToOneField
+        to determine its data type.
+        """
+        return self.db_type(connection)
+
     def db_parameters(self, connection):
         """
         Extension of db_type(), providing a range of different return
@@ -960,6 +968,9 @@ class AutoField(Field):
                 params={'value': value},
             )
 
+    def rel_db_type(self, connection):
+        return IntegerField().db_type(connection=connection)
+
     def validate(self, value, model_instance):
         pass
 
@@ -2072,7 +2083,24 @@ class NullBooleanField(Field):
         return super(NullBooleanField, self).formfield(**defaults)
 
 
-class PositiveIntegerField(IntegerField):
+class PositiveIntegerRelDbTypeMixin(object):
+
+    def rel_db_type(self, connection):
+        """
+        Return the data type that a related field pointing to this field should
+        use. In most cases, a foreign key pointing to a positive integer
+        primary key will have an integer column data type but some databases
+        (e.g. MySQL) have an unsigned integer type. In that case
+        (related_fields_match_type=True), the primary key should return its
+        db_type.
+        """
+        if connection.features.related_fields_match_type:
+            return self.db_type(connection)
+        else:
+            return IntegerField().db_type(connection=connection)
+
+
+class PositiveIntegerField(PositiveIntegerRelDbTypeMixin, IntegerField):
     description = _("Positive integer")
 
     def get_internal_type(self):
@@ -2084,7 +2112,7 @@ class PositiveIntegerField(IntegerField):
         return super(PositiveIntegerField, self).formfield(**defaults)
 
 
-class PositiveSmallIntegerField(IntegerField):
+class PositiveSmallIntegerField(PositiveIntegerRelDbTypeMixin, IntegerField):
     description = _("Positive small integer")
 
     def get_internal_type(self):

django/db/models/fields/related.py

@@ -18,10 +18,7 @@ from django.utils.functional import cached_property, curry
 from django.utils.translation import ugettext_lazy as _
 from django.utils.version import get_docs_version
 
-from . import (
-    AutoField, Field, IntegerField, PositiveIntegerField,
-    PositiveSmallIntegerField,
-)
+from . import Field
 from .related_descriptors import (
     ForwardManyToOneDescriptor, ManyToManyDescriptor,
     ReverseManyToOneDescriptor, ReverseOneToOneDescriptor,
@@ -935,19 +932,7 @@ class ForeignKey(ForeignObject):
         return super(ForeignKey, self).formfield(**defaults)
 
     def db_type(self, connection):
-        # The database column type of a ForeignKey is the column type
-        # of the field to which it points. An exception is if the ForeignKey
-        # points to an AutoField/PositiveIntegerField/PositiveSmallIntegerField,
-        # in which case the column type is simply that of an IntegerField.
-        # If the database needs similar types for key fields however, the only
-        # thing we can do is making AutoField an IntegerField.
-        rel_field = self.target_field
-        if (isinstance(rel_field, AutoField) or
-                (not connection.features.related_fields_match_type and
-                isinstance(rel_field, (PositiveIntegerField,
-                                       PositiveSmallIntegerField)))):
-            return IntegerField().db_type(connection=connection)
-        return rel_field.db_type(connection=connection)
+        return self.target_field.rel_db_type(connection=connection)
 
     def db_parameters(self, connection):
         return {"type": self.db_type(connection), "check": []}