Fix named CHECK constraint name omitted on repeated creates

Fixed issue where the CHECK constraint generated by :class:`_types.Boolean`
or :class:`_types.Enum` would fail to render the naming convention
correctly after the first compilation, due to an unintended change of state
within the name given to the constraint. This issue was first introduced in
0.9 in the fix for issue #3067, and the fix revises the approach taken at
that time which appears to have been more involved than what was needed.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #6007
Change-Id: I7ecff0a9d86191520f6841b3922a5af5a6971fba

4 files changed, 12 insertions, 41 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index c6fa6072e..d48bca14e 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -4902,16 +4902,13 @@ class IdentifierPreparer(object):
     def format_constraint(self, constraint, _alembic_quote=True):
         naming = util.preloaded.sql_naming
 
-        if isinstance(constraint.name, elements._defer_name):
+        if constraint.name is elements._NONE_NAME:
             name = naming._constraint_name_for_table(
                 constraint, constraint.table
             )
 
             if name is None:
-                if isinstance(constraint.name, elements._defer_none_name):
-                    return None
-                else:
-                    name = constraint.name
+                return None
         else:
             name = constraint.name
 
diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index 85200bf25..b26918f2f 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -4979,33 +4979,8 @@ class conv(_truncated_label):
     __slots__ = ()
 
 
-class _defer_name(_truncated_label):
-    """mark a name as 'deferred' for the purposes of automated name
-    generation.
-
-    """
-
-    __slots__ = ()
-
-    def __new__(cls, value):
-        if value is None:
-            return _NONE_NAME
-        elif isinstance(value, conv):
-            return value
-        else:
-            return super(_defer_name, cls).__new__(cls, value)
-
-    def __reduce__(self):
-        return self.__class__, (util.text_type(self),)
-
-
-class _defer_none_name(_defer_name):
-    """indicate a 'deferred' name that was ultimately the value None."""
-
-    __slots__ = ()
-
-
-_NONE_NAME = _defer_none_name("_unnamed_")
+_NONE_NAME = util.symbol("NONE_NAME")
+"""indicate a 'deferred' name that was ultimately the value None."""
 
 # for backwards compatibility in case
 # someone is re-implementing the
diff --git a/lib/sqlalchemy/sql/naming.py b/lib/sqlalchemy/sql/naming.py
index 005c41683..b79316435 100644
--- a/lib/sqlalchemy/sql/naming.py
+++ b/lib/sqlalchemy/sql/naming.py
@@ -13,8 +13,7 @@
 import re
 
 from . import events  # noqa
-from .elements import _defer_name
-from .elements import _defer_none_name
+from .elements import _NONE_NAME
 from .elements import conv
 from .schema import CheckConstraint
 from .schema import Column
@@ -57,7 +56,7 @@ class ConventionDict(object):
                 return getattr(col, attrname)
 
     def _key_constraint_name(self):
-        if isinstance(self._const_name, (type(None), _defer_none_name)):
+        if self._const_name in (None, _NONE_NAME):
             raise exc.InvalidRequestError(
                 "Naming convention including "
                 "%(constraint_name)s token requires that "
@@ -160,14 +159,14 @@ def _constraint_name_for_table(const, table):
         and (
             const.name is None
             or "constraint_name" in convention
-            or isinstance(const.name, _defer_name)
+            or const.name is _NONE_NAME
         )
     ):
         return conv(
             convention
             % ConventionDict(const, table, metadata.naming_convention)
         )
-    elif isinstance(convention, _defer_none_name):
+    elif convention is _NONE_NAME:
         return None
 
 
@@ -203,7 +202,7 @@ def _constraint_name(const, table):
         )
 
     elif isinstance(table, Table):
-        if isinstance(const.name, (conv, _defer_name)):
+        if isinstance(const.name, conv) or const.name is _NONE_NAME:
             return
 
         newname = _constraint_name_for_table(const, table)
diff --git a/lib/sqlalchemy/sql/sqltypes.py b/lib/sqlalchemy/sql/sqltypes.py
index 3484b3ac8..d075ef77d 100644
--- a/lib/sqlalchemy/sql/sqltypes.py
+++ b/lib/sqlalchemy/sql/sqltypes.py
@@ -22,7 +22,7 @@ from . import type_api
 from .base import _bind_or_error
 from .base import NO_ARG
 from .base import SchemaEventTarget
-from .elements import _defer_name
+from .elements import _NONE_NAME
 from .elements import quoted_name
 from .elements import Slice
 from .elements import TypeCoerce as type_coerce  # noqa
@@ -1659,7 +1659,7 @@ class Enum(Emulated, String, SchemaType):
 
         e = schema.CheckConstraint(
             type_coerce(column, self).in_(self.enums),
-            name=_defer_name(self.name),
+            name=_NONE_NAME if self.name is None else self.name,
             _create_rule=util.portable_instancemethod(
                 self._should_create_constraint,
                 {"variant_mapping": variant_mapping},
@@ -1868,7 +1868,7 @@ class Boolean(Emulated, TypeEngine, SchemaType):
 
         e = schema.CheckConstraint(
             type_coerce(column, self).in_([0, 1]),
-            name=_defer_name(self.name),
+            name=_NONE_NAME if self.name is None else self.name,
             _create_rule=util.portable_instancemethod(
                 self._should_create_constraint,
                 {"variant_mapping": variant_mapping},