Implement support for functions as FROM with columns clause support

WIP

Fixes: #3566
Change-Id: I5b093b72533ef695293e737eb75850b9713e5e03

1 files changed, 21 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/coercions.py b/lib/sqlalchemy/sql/coercions.py
index 05e0a4fcf..b5b5082e6 100644
--- a/lib/sqlalchemy/sql/coercions.py
+++ b/lib/sqlalchemy/sql/coercions.py
@@ -431,6 +431,20 @@ class ExpressionElementImpl(_ColumnCoercions, RoleImpl):
             except exc.ArgumentError as err:
                 self._raise_for_expected(element, err=err)
 
+    def _raise_for_expected(self, element, argname=None, resolved=None, **kw):
+        if isinstance(element, roles.AnonymizedFromClauseRole):
+            advice = (
+                "To create a "
+                "column expression from a FROM clause row "
+                "as a whole, use the .record() method."
+            )
+        else:
+            advice = None
+
+        return super(ExpressionElementImpl, self)._raise_for_expected(
+            element, argname=argname, resolved=resolved, advice=advice, **kw
+        )
+
 
 class BinaryElementImpl(ExpressionElementImpl, RoleImpl):
 
@@ -597,6 +611,13 @@ class ColumnArgumentOrKeyImpl(_ReturnsStringKey, RoleImpl):
     __slots__ = ()
 
 
+class StrAsPlainColumnImpl(_CoerceLiterals, RoleImpl):
+    __slots__ = ()
+
+    def _text_coercion(self, element, argname=None):
+        return elements.ColumnClause(element)
+
+
 class ByOfImpl(_CoerceLiterals, _ColumnCoercions, RoleImpl, roles.ByOfRole):
 
     __slots__ = ()