Allow dialect-specific stringification

Dialect-specific constructs such as
:meth:`_postgresql.Insert.on_conflict_do_update` can now stringify in-place
without the need to specify an explicit dialect object.  The constructs,
when called upon for ``str()``, ``print()``, etc. now have internal
direction to call upon their appropriate dialect rather than the
"default"dialect which doesn't know how to stringify these.   The approach
is also adapted to generic schema-level create/drop such as
:class:`_schema.AddConstraint`, which will adapt its stringify dialect to
one indicated by the element within it, such as the
:class:`_postgresql.ExcludeConstraint` object.

mostly towards being able to provide doctest-style
examples for "on conflict" constructs using print statements.

Change-Id: I4b855516fe6dee2df77744c1bb21a373d7fbab93

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/ext.py b/lib/sqlalchemy/dialects/postgresql/ext.py
index c139fe94f..78d9a96b6 100644
--- a/lib/sqlalchemy/dialects/postgresql/ext.py
+++ b/lib/sqlalchemy/dialects/postgresql/ext.py
@@ -52,6 +52,8 @@ class aggregate_order_by(expression.ColumnElement):
 
     __visit_name__ = "aggregate_order_by"
 
+    stringify_dialect = "postgresql"
+
     def __init__(self, target, *order_by):
         self.target = coercions.expect(roles.ExpressionElementRole, target)
 
@@ -96,6 +98,8 @@ class ExcludeConstraint(ColumnCollectionConstraint):
 
     where = None
 
+    create_drop_stringify_dialect = "postgresql"
+
     @elements._document_text_coercion(
         "where",
         ":class:`.ExcludeConstraint`",