Expand joins when calculating PostgreSQL "WITH FOR UPDATE OF"

Modified the :paramref:`.Select.with_for_update.of` parameter so that if a
join or other composed selectable is passed, the individual :class:`.Table`
objects will be filtered from it, allowing one to pass a join() object to
the parameter, as occurs normally when using joined table inheritance with
the ORM.  Pull request courtesy Raymond Lu.

Fixes: #4550
Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Closes: #4551
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4551
Pull-request-sha: 452da77d154a4087d530456db1c9af207d65cef4

Change-Id: If4b7c231f7b71190d7245543959fb5c3351125a1

1 files changed, 6 insertions, 4 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index 4d302dabe..3781a7ba2 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -929,6 +929,7 @@ from ...sql import compiler
 from ...sql import elements
 from ...sql import expression
 from ...sql import sqltypes
+from ...sql import util as sql_util
 from ...types import BIGINT
 from ...types import BOOLEAN
 from ...types import CHAR
@@ -1681,10 +1682,11 @@ class PGCompiler(compiler.SQLCompiler):
             tmp = " FOR UPDATE"
 
         if select._for_update_arg.of:
-            tables = util.OrderedSet(
-                c.table if isinstance(c, expression.ColumnClause) else c
-                for c in select._for_update_arg.of
-            )
+
+            tables = util.OrderedSet()
+            for c in select._for_update_arg.of:
+                tables.update(sql_util.surface_selectables_only(c))
+
             tmp += " OF " + ", ".join(
                 self.process(table, ashint=True, use_schema=False, **kw)
                 for table in tables