在不可为空的列中忽略默认值

我正在尝试使用以下 SQL alchemy 脚本在表中创建一个新的布尔型、不可空列（默认值=True）：

from sqlalchemy import MetaData, Table, Boolean, Column

def upgrade(migrate_engine):
    meta = MetaData(bind=migrate_engine)
    message_table = Table('message', meta, autoload=True)
    col = Column('include_signature', Boolean(), default=True, nullable=False)
    col.create(message_table)

这会导致一个相当令人费解的错误：

  File "src/adhocracy/migration/versions/061_optional_massmessage_signature.py", line 7, in upgrade
    col.create(message_table)
  File "eggs/sqlalchemy_migrate-0.7.2-py2.6.egg/migrate/changeset/schema.py", line 528, in create
    engine._run_visitor(visitorcallable, self, connection, **kwargs)
  File "eggs/SQLAlchemy-0.7.10-py2.6-linux-x86_64.egg/sqlalchemy/engine/base.py", line 2302, in _run_visitor
    conn._run_visitor(visitorcallable, element, **kwargs)
  File "eggs/SQLAlchemy-0.7.10-py2.6-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1972, in _run_visitor
    **kwargs).traverse_single(element)
  File "eggs/sqlalchemy_migrate-0.7.2-py2.6.egg/migrate/changeset/ansisql.py", line 53, in traverse_single
    ret = super(AlterTableVisitor, self).traverse_single(elem)
  File "eggs/SQLAlchemy-0.7.10-py2.6-linux-x86_64.egg/sqlalchemy/sql/visitors.py", line 106, in traverse_single
    return meth(obj, **kw)
  File "eggs/sqlalchemy_migrate-0.7.2-py2.6.egg/migrate/changeset/databases/sqlite.py", line 75, in visit_column
    super(SQLiteColumnGenerator,self).visit_column(column)
  File "eggs/sqlalchemy_migrate-0.7.2-py2.6.egg/migrate/changeset/ansisql.py", line 101, in visit_column
    self.execute()
  File "eggs/sqlalchemy_migrate-0.7.2-py2.6.egg/migrate/changeset/ansisql.py", line 42, in execute
    return self.connection.execute(self.buffer.getvalue())
  File "eggs/SQLAlchemy-0.7.10-py2.6-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1449, in execute
    params)
  File "eggs/SQLAlchemy-0.7.10-py2.6-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1628, in _execute_text
    statement, parameters
  File "eggs/SQLAlchemy-0.7.10-py2.6-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1698, in _execute_context
    context)
  File "eggs/SQLAlchemy-0.7.10-py2.6-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1691, in _execute_context
    context)
  File "eggs/SQLAlchemy-0.7.10-py2.6-linux-x86_64.egg/sqlalchemy/engine/default.py", line 331, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.OperationalError: (OperationalError) Cannot add a NOT NULL column
with default value NULL '\nALTER TABLE message ADD include_signature BOOLEAN NOT NULL' ()

据我了解，sqlalchemy 缺少默认值，但我确实包括了default=True！这里有什么问题吗？

Use server_default="true"， 像那样：

col = Column('include_signature', Boolean(), default=True,
             server_default="true", nullable=False)

Where default用于类实例并且server_default用于数据库列定义。