Invoke column_expression() for subsequent SELECTs in CompoundSelect

Fixed bug where :meth:`.TypeEngine.column_expression` method would not be
applied to subsequent SELECT statements inside of a UNION or other
:class:`.CompoundSelect`, even though the SELECT statements are rendered at
the topmost level of the statement.   New logic now differentiates between
rendering the column expression, which is needed for all SELECTs in the
list, vs. gathering the returned data type for the result row, which is
needed only for the first SELECT.

Fixes: #4787
Change-Id: Iceb63e430e76d2365649aa25ead09c4e2a062e10

1 files changed, 19 insertions, 10 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index 400ac2749..8bd2249e2 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -1799,18 +1799,26 @@ class SQLCompiler(Compiled):
         column_clause_args,
         name=None,
         within_columns_clause=True,
+        need_column_expressions=False,
     ):
         """produce labeled columns present in a select()."""
 
         impl = column.type.dialect_impl(self.dialect)
-        if impl._has_column_expression and populate_result_map:
+
+        if impl._has_column_expression and (
+            need_column_expressions or populate_result_map
+        ):
             col_expr = impl.column_expression(column)
 
-            def add_to_result_map(keyname, name, objects, type_):
-                self._add_to_result_map(
-                    keyname, name, (column,) + objects, type_
-                )
+            if populate_result_map:
 
+                def add_to_result_map(keyname, name, objects, type_):
+                    self._add_to_result_map(
+                        keyname, name, (column,) + objects, type_
+                    )
+
+            else:
+                add_to_result_map = None
         else:
             col_expr = column
             if populate_result_map:
@@ -2085,15 +2093,15 @@ class SQLCompiler(Compiled):
         toplevel = not self.stack
         entry = self._default_stack_entry if toplevel else self.stack[-1]
 
-        populate_result_map = (
+        populate_result_map = need_column_expressions = (
             toplevel
-            or (
-                compound_index == 0
-                and entry.get("need_result_map_for_compound", False)
-            )
+            or entry.get("need_result_map_for_compound", False)
             or entry.get("need_result_map_for_nested", False)
         )
 
+        if compound_index > 0:
+            populate_result_map = False
+
         # this was first proposed as part of #3372; however, it is not
         # reached in current tests and could possibly be an assertion
         # instead.
@@ -2138,6 +2146,7 @@ class SQLCompiler(Compiled):
                     asfrom,
                     column_clause_args,
                     name=name,
+                    need_column_expressions=need_column_expressions,
                 )
                 for name, column in select._columns_plus_names
             ]