title: Flask-SQLAlchemy mixin with CheckConstraint in __table_args__ results with errors
body: <p>I'm trying to create base Model with single CheckConstraint in base class. When I mixin the base class into flask-sqlalchemy model and try to use it, I get "Can't add unnamed column to column collection".</p>
  
  <p>I tried to play with different ways of declaring this CheckConstraint 
  but all resulted with this same error. Any ideas what's causing this?</p>
  
  <pre><code>from sqlalchemy.ext.declarative import declared_attr
  from myproject.db import db
  
  
  class FilteringMixin(db.Model):
      __abstract__ = True
  
      FILTER_TYPES = {
          'whitelist': 0,
          'blacklist': 1,
      }
  
      @declared_attr.cascading
      def filter_type(cls):
          return db.Column(db.SmallInteger, nullable=False,
                           default=cls.FILTER_TYPES['whitelist'])
  
      @declared_attr.cascading
      def __table_args__(cls):
          return (
                   db.CheckConstraint(cls.filter_type.in_(cls.FILTER_TYPES.values())),
          )
  
  
  class FilteredConnectionType(FilteringMixin, db.Model):
      CONNECTION_TYPES = {
          'wifi': 0,
          'cellular': 1,
      }
  
      __tablename__ = 'filtered_connection_types'
  
      id = db.Column(db.BigInteger, primary_key=True)
  
      connection_type = db.Column(db.SmallInteger, nullable=False,
                                  default=CONNECTION_TYPES['cellular'])
  
      @declared_attr.cascading
      def __table_args__(cls):
          return (
              db.CheckConstraint(cls.connection_type.in_(cls.CONNECTION_TYPES.values())),
          ) + FilteringMixin.__table_args__
  </code></pre>
  
  <p>After that, trying to use it I get: </p>
  
  <pre><code>python manage.py shell  
  ~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/api.py:173: SAWarning: Unmanaged access of declarative attribute __table_args__ from non-mapped class FilteringMixin
    (desc.fget.__name__, cls.__name__))
  ~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/api.py:173: SAWarning: Unmanaged access of declarative attribute filter_type from non-mapped class FilteringMixin
    (desc.fget.__name__, cls.__name__))
  Traceback (most recent call last):
    File "manage.py", line 4, in &lt;module&gt;
      from myproject import create_app, environments, scripts
    File "~/myuser/myproject/myproject/__init__.py", line 9, in &lt;module&gt;
      import models
    File "~/myuser/myproject/myproject/models/__init__.py", line 13, in &lt;module&gt;
      from .filtering_implemented import FilteredConnectionType
    File "~/myuser/myproject/myproject/models/filtering_implemented.py", line 6, in &lt;module&gt;
      class FilteredConnectionType(FilteringMixin, db.Model):
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/flask_sqlalchemy/__init__.py", line 609, in __init__
      DeclarativeMeta.__init__(self, name, bases, d)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/api.py", line 55, in __init__
      _as_declarative(cls, classname, cls.__dict__)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 88, in _as_declarative
      _MapperConfig.setup_mapping(cls, classname, dict_)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 103, in setup_mapping
      cfg_cls(cls_, classname, dict_)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 131, in __init__
      self._setup_table()
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/ext/declarative/base.py", line 394, in _setup_table
      **table_kw)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/sql/schema.py", line 416, in __new__
      metadata._remove_table(name, schema)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 60, in __exit__
      compat.reraise(exc_type, exc_value, exc_tb)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/sql/schema.py", line 411, in __new__
      table._init(name, metadata, *args, **kw)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/sql/schema.py", line 488, in _init
      self._init_items(*args)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/sql/schema.py", line 72, in _init_items
      item._set_parent_with_dispatch(self)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/sql/base.py", line 433, in _set_parent_with_dispatch
      self._set_parent(parent)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/sql/schema.py", line 2553, in _set_parent
      ColumnCollectionMixin._set_parent(self, table)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/sql/schema.py", line 2521, in _set_parent
      self.columns.add(col)
    File "~/myuser/myproject/.venv/local/lib/python2.7/site-packages/sqlalchemy/sql/base.py", line 509, in add
      "Can't add unnamed column to column collection")
  sqlalchemy.exc.ArgumentError: Can't add unnamed column to column collection
  </code></pre>