Remove "subclass existing types" use case

Thanks to :ref:`change_3981`, we no longer need to rely on recipes that
subclass dialect-specific types directly, :class:`.TypeDecorator` can now
handle all cases.   Additionally, the above change made it slightly less
likely that a direct subclass of a base SQLAlchemy type would work as
expected, which could be misleading.  Documentation has been updated to use
:class:`.TypeDecorator` for these examples including the PostgreSQL
"ArrayOfEnum" example datatype and direct support for the "subclass a type
directly" has been removed.

Fixes: #4580
Change-Id: I866f246cccc736ea618dc965ab3604762f7a52fe

1 files changed, 9 insertions, 2 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index 3781a7ba2..ceb624644 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -857,9 +857,16 @@ Using ENUM with ARRAY
 
 The combination of ENUM and ARRAY is not directly supported by backend
 DBAPIs at this time.   In order to send and receive an ARRAY of ENUM,
-use the following workaround type::
+use the following workaround type, which decorates the
+:class:`.postgresql.ARRAY` datatype.
 
-    class ArrayOfEnum(ARRAY):
+.. sourcecode:: python
+
+    from sqlalchemy import TypeDecorator
+    from sqlalchemy.dialects.postgresql import ARRAY
+
+    class ArrayOfEnum(TypeDecorator):
+        impl = ARRAY
 
         def bind_expression(self, bindvalue):
             return sa.cast(bindvalue, self)