Don't reorder PrimaryKeyConstraint columns if explicit

Dialed back the "order the primary key columns per auto-increment"
described in :ref:`change_mysql_3216` a bit, so that if the
:class:`.PrimaryKeyConstraint` is explicitly defined, the order
of columns is maintained exactly, allowing control of this behavior
when necessary.

Change-Id: I9e7902c57a96c15968a6abf53e319acf15680da0
Fixes: #3726

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index cb01a49e3..ee139827a 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -475,7 +475,8 @@ class Table(DialectKWArgs, SchemaItem, TableClause):
         self.indexes = set()
         self.constraints = set()
         self._columns = ColumnCollection()
-        PrimaryKeyConstraint()._set_parent_with_dispatch(self)
+        PrimaryKeyConstraint(_implicit_generated=True).\
+            _set_parent_with_dispatch(self)
         self.foreign_keys = set()
         self._extra_dependencies = set()
         if self.schema is not None:
@@ -3023,6 +3024,10 @@ class PrimaryKeyConstraint(ColumnCollectionConstraint):
 
     __visit_name__ = 'primary_key_constraint'
 
+    def __init__(self, *columns, **kw):
+        self._implicit_generated = kw.pop('_implicit_generated', False)
+        super(PrimaryKeyConstraint, self).__init__(*columns, **kw)
+
     def _set_parent(self, table):
         super(PrimaryKeyConstraint, self)._set_parent(table)