Make sure we only create the minimum number of table indexes for MySQL.

This patch simplifies a bunch of code for all backends and removes some
duplicate index creation for MySQL, in particular (versions 4.x and later).
Patch from Nis Jørgensen.

Fixed #5671, #5680, #7170, #7186.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7790 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/fields/__init__.py

@@ -91,7 +91,7 @@ class Field(object):
         self.name = name
         self.verbose_name = verbose_name
         self.primary_key = primary_key
-        self.max_length, self.unique = max_length, unique
+        self.max_length, self._unique = max_length, unique
         self.blank, self.null = blank, null
         # Oracle treats the empty string ('') as null, so coerce the null
         # option whenever '' is a possible value.
@@ -168,6 +168,10 @@ class Field(object):
         except KeyError:
             return None
 
+    def unique(self):
+        return self._unique or self.primary_key
+    unique = property(unique)
+
     def validate_full(self, field_data, all_data):
         """
         Returns a list of errors for this field. This is the main interface,