Check for __clause_element__() in ORM insert/update

ORM attributes can now be assigned any object that is has a
``__clause_element__()`` attribute, which will result in inline
SQL the way any :class:`.ClauseElement` class does.  This covers other
mapped attributes not otherwise transformed by further expression
constructs.

As part of this, it was considered that we could add
__clause_element__() to ClauseElement, however this causes endless loops
in a "while" pattern and this pattern has been identified in third
party libraries.  Add a test to ensure we never make that change.

Change-Id: I9e15b3f1c4883fd3909acbf7dc81d034c6e3ce1d
Fixes: #3802

2 files changed, 9 insertions, 5 deletions

diff --git a/lib/sqlalchemy/ext/hybrid.py b/lib/sqlalchemy/ext/hybrid.py
index 2f473fd31..99f938ebb 100644
--- a/lib/sqlalchemy/ext/hybrid.py
+++ b/lib/sqlalchemy/ext/hybrid.py
@@ -809,7 +809,7 @@ class Comparator(interfaces.PropComparator):
 
     def __clause_element__(self):
         expr = self.expression
-        while hasattr(expr, '__clause_element__'):
+        if hasattr(expr, '__clause_element__'):
             expr = expr.__clause_element__()
         return expr
 
diff --git a/lib/sqlalchemy/orm/persistence.py b/lib/sqlalchemy/orm/persistence.py
index 56b028375..24a33ee8d 100644
--- a/lib/sqlalchemy/orm/persistence.py
+++ b/lib/sqlalchemy/orm/persistence.py
@@ -388,8 +388,10 @@ def _collect_insert_commands(
             col = propkey_to_col[propkey]
             if value is None and propkey not in eval_none and not render_nulls:
                 continue
-            elif not bulk and isinstance(value, sql.ClauseElement):
-                value_params[col.key] = value
+            elif not bulk and hasattr(value, '__clause_element__') or \
+                    isinstance(value, sql.ClauseElement):
+                value_params[col.key] = value.__clause_element__() \
+                    if hasattr(value, '__clause_element__') else value
             else:
                 params[col.key] = value
 
@@ -462,8 +464,10 @@ def _collect_update_commands(
                 value = state_dict[propkey]
                 col = propkey_to_col[propkey]
 
-                if isinstance(value, sql.ClauseElement):
-                    value_params[col] = value
+                if hasattr(value, '__clause_element__') or \
+                        isinstance(value, sql.ClauseElement):
+                    value_params[col] = value.__clause_element__() \
+                        if hasattr(value, '__clause_element__') else value
                 # guard against values that generate non-__nonzero__
                 # objects for __eq__()
                 elif state.manager[propkey].impl.is_equal(