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

2 files changed, 22 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
diff --git a/lib/sqlalchemy/sql/util.py b/lib/sqlalchemy/sql/util.py
index 5a44f873d..3077840c6 100644
--- a/lib/sqlalchemy/sql/util.py
+++ b/lib/sqlalchemy/sql/util.py
@@ -29,11 +29,13 @@ from .elements import ColumnElement
 from .elements import Null
 from .elements import UnaryExpression
 from .schema import Column
+from .selectable import Alias
 from .selectable import FromClause
 from .selectable import FromGrouping
 from .selectable import Join
 from .selectable import ScalarSelect
 from .selectable import SelectBase
+from .selectable import TableClause
 from .. import exc
 from .. import util
 
@@ -339,6 +341,20 @@ def surface_selectables(clause):
             stack.append(elem.element)
 
 
+def surface_selectables_only(clause):
+    stack = [clause]
+    while stack:
+        elem = stack.pop()
+        if isinstance(elem, (TableClause, Alias)):
+            yield elem
+        if isinstance(elem, Join):
+            stack.extend((elem.left, elem.right))
+        elif isinstance(elem, FromGrouping):
+            stack.append(elem.element)
+        elif isinstance(elem, ColumnClause):
+            stack.append(elem.table)
+
+
 def surface_column_elements(clause, include_scalar_selects=True):
     """traverse and yield only outer-exposed column elements, such as would
     be addressable in the WHERE clause of a SELECT if this element were