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, 16 insertions, 0 deletions

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