Merge "Add default expression to query_expression()"

2 files changed, 33 insertions, 10 deletions

diff --git a/lib/sqlalchemy/orm/__init__.py b/lib/sqlalchemy/orm/__init__.py
index 110c27811..fabb095a2 100644
--- a/lib/sqlalchemy/orm/__init__.py
+++ b/lib/sqlalchemy/orm/__init__.py
@@ -180,9 +180,22 @@ def deferred(*columns, **kw):
     return ColumnProperty(deferred=True, *columns, **kw)
 
 
-def query_expression():
+def query_expression(default_expr=_sql.null()):
     """Indicate an attribute that populates from a query-time SQL expression.
 
+    :param default_expr: Optional SQL expression object that will be used in
+        all cases if not assigned later with :func:`_orm.with_expression`.
+        E.g.::
+
+            from sqlalchemy.sql import literal
+
+            class C(Base):
+                #...
+                my_expr = query_expression(literal(1))
+
+        .. versionadded:: 1.3.18
+
+
     .. versionadded:: 1.2
 
     .. seealso::
@@ -190,7 +203,7 @@ def query_expression():
         :ref:`mapper_querytime_expression`
 
     """
-    prop = ColumnProperty(_sql.null())
+    prop = ColumnProperty(default_expr)
     prop.strategy_key = (("query_expression", True),)
     return prop
 
diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index 2b8c384c9..f67c23aab 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -245,6 +245,11 @@ class ExpressionColumnLoader(ColumnLoader):
     def __init__(self, parent, strategy_key):
         super(ExpressionColumnLoader, self).__init__(parent, strategy_key)
 
+        null = sql.null()
+        self._have_default_expression = any(
+            not c.compare(null) for c in self.parent_property.columns
+        )
+
     def setup_query(
         self,
         compile_state,
@@ -256,19 +261,24 @@ class ExpressionColumnLoader(ColumnLoader):
         memoized_populators,
         **kwargs
     ):
-
+        columns = None
         if loadopt and "expression" in loadopt.local_opts:
             columns = [loadopt.local_opts["expression"]]
+        elif self._have_default_expression:
+            columns = self.parent_property.columns
 
-            for c in columns:
-                if adapter:
-                    c = adapter.columns[c]
-                column_collection.append(c)
+        if columns is None:
+            return
 
-            fetch = columns[0]
+        for c in columns:
             if adapter:
-                fetch = adapter.columns[fetch]
-            memoized_populators[self.parent_property] = fetch
+                c = adapter.columns[c]
+            column_collection.append(c)
+
+        fetch = columns[0]
+        if adapter:
+            fetch = adapter.columns[fetch]
+        memoized_populators[self.parent_property] = fetch
 
     def create_row_processor(
         self, context, path, loadopt, mapper, result, adapter, populators