Use schema._copy_expression() fully in column collection constraints

Fixed issue where using :meth:`_schema.Table.to_metadata` (called
:meth:`_schema.Table.tometadata` in 1.3) in conjunction with a PostgreSQL
:class:`_postgresql.ExcludeConstraint` that made use of ad-hoc column
expressions would fail to copy correctly.

Fixes: #5850
Change-Id: I062480afb23f6f60962b7b55bc93f5e4e6ff05e4

1 files changed, 9 insertions, 2 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/ext.py b/lib/sqlalchemy/dialects/postgresql/ext.py
index 1bd1e5258..a93a3477d 100644
--- a/lib/sqlalchemy/dialects/postgresql/ext.py
+++ b/lib/sqlalchemy/dialects/postgresql/ext.py
@@ -12,6 +12,7 @@ from ...sql import elements
 from ...sql import expression
 from ...sql import functions
 from ...sql import roles
+from ...sql import schema
 from ...sql.schema import ColumnCollectionConstraint
 
 
@@ -240,8 +241,14 @@ class ExcludeConstraint(ColumnCollectionConstraint):
             )
         ]
 
-    def copy(self, **kw):
-        elements = [(col, self.operators[col]) for col in self.columns.keys()]
+    def copy(self, target_table=None, **kw):
+        elements = [
+            (
+                schema._copy_expression(expr, self.parent, target_table),
+                self.operators[expr.name],
+            )
+            for expr in self.columns
+        ]
         c = self.__class__(
             *elements,
             name=self.name,