omit text from selected_columns; clear memoizations

Fixed regression where usage of the :func:`_sql.text` construct inside the
columns clause of a :class:`_sql.Select` construct, which is better handled
by using a :func:`_sql.literal_column` construct, would nonetheless prevent
constructs like :func:`_sql.union` from working correctly. Other use cases,
such as constructing subuqeries, continue to work the same as in prior
versions where the :func:`_sql.text` construct is silently omitted from the
collection of exported columns. Also repairs similar use within the
ORM.

This adds a new internal method _all_selected_columns.  The existing
"selected_columns" collection can't store a TextClause and this never
worked, so they are omitted.  The TextClause is also not "exported",
i.e. available for SELECT from a subquery, as was already the case
in 1.3, so the "exported_columns" and "exported_columns_iterator"
accessors are where we now omit TextClause.

Fixed regression involving legacy methods such as
:meth:`_sql.Select.append_column` where internal assertions would fail.

Fixes: #6343
Fixes: #6261
Change-Id: I7c2e5b9ae5d94131c77599a020f4310dcf812bcf

2 files changed, 14 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/__init__.py b/lib/sqlalchemy/testing/__init__.py
index a3ce24226..a311efa74 100644
--- a/lib/sqlalchemy/testing/__init__.py
+++ b/lib/sqlalchemy/testing/__init__.py
@@ -60,6 +60,7 @@ from .exclusions import only_if
 from .exclusions import only_on
 from .exclusions import skip
 from .exclusions import skip_if
+from .schema import eq_clause_element
 from .schema import eq_type_affinity
 from .util import adict
 from .util import fail
diff --git a/lib/sqlalchemy/testing/schema.py b/lib/sqlalchemy/testing/schema.py
index fee021cff..ede3702e5 100644
--- a/lib/sqlalchemy/testing/schema.py
+++ b/lib/sqlalchemy/testing/schema.py
@@ -156,6 +156,19 @@ class eq_type_affinity(object):
         return self.target._type_affinity is not other._type_affinity
 
 
+class eq_clause_element(object):
+    """Helper to compare SQL structures based on compare()"""
+
+    def __init__(self, target):
+        self.target = target
+
+    def __eq__(self, other):
+        return self.target.compare(other)
+
+    def __ne__(self, other):
+        return not self.target.compare(other)
+
+
 def _truncate_name(dialect, name):
     if len(name) > dialect.max_identifier_length:
         return (