Flask-SQLAlchemy mixin 与 __table_args__ 中的 CheckConstraint 导致错误答案

【问题标题】：Flask-SQLAlchemy mixin with CheckConstraint in __table_args__ results with errorsFlask-SQLAlchemy mixin 与 __table_args__ 中的 CheckConstraint 导致错误
【发布时间】：2016-01-24 09:16:44
【问题描述】：

我正在尝试在基类中使用单个 CheckConstraint 创建基本模型。当我将基类混入flask-sqlalchemy模型并尝试使用它时，我得到“无法将未命名的列添加到列集合”。

我尝试使用不同的方式声明此 CheckConstraint 但都导致了同样的错误。有什么想法是什么原因造成的吗？

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__

之后，尝试使用它我得到：

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 <module>
    from myproject import create_app, environments, scripts
  File "~/myuser/myproject/myproject/__init__.py", line 9, in <module>
    import models
  File "~/myuser/myproject/myproject/models/__init__.py", line 13, in <module>
    from .filtering_implemented import FilteredConnectionType
  File "~/myuser/myproject/myproject/models/filtering_implemented.py", line 6, in <module>
    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

【问题讨论】：

FilteringMixin 继承自 do.Model。 FilteringConnectionType 不需要从两者继承。

标签： flask sqlalchemy flask-sqlalchemy

【解决方案1】：

我认为真正的问题在错误输出顶部的两个警告中突出显示：

.../api.py:173: SAWarning: Unmanaged access of declarative attribute __table_args__ from non-mapped class FilteringMixin
  (desc.fget.__name__, cls.__name__))

据我了解，这是由FilteringMixin.__table_args__ 使用引起的，解决方法是使用基于字符串的语法来描述约束：

class FilteringMixin(object):
    FILTER_TYPES = {
        'whitelist': 0,
        'blacklist': 1,
    }

    @declared_attr
    def filter_type(cls):
        return db.Column(db.SmallInteger, nullable=False,
                         default=cls.FILTER_TYPES['whitelist'])

    @declared_attr
    def __table_args__(cls):
        return (
             db.CheckConstraint(
                 'filter_type in (%s)' %
                 ','.join(str(t) for t in cls.FILTER_TYPES.values())),
        )


class FilteredConnectionType(FilteringMixin, db.Model):
    CONNECTION_TYPES = {
        'wifi': 0,
        'cellular': 1,
    }

    __tablename__ = 'filtered_connection_types'
    # Changed to db.Integer from db.BigInteger because of SQLite
    id = db.Column(db.Integer, primary_key=True)
    connection_type = db.Column(db.SmallInteger, nullable=False,
                                default=CONNECTION_TYPES['cellular'])

    @declared_attr
    def __table_args__(cls):
        return (
            db.CheckConstraint(cls.connection_type.in_(cls.CONNECTION_TYPES.values())),
        ) + FilteringMixin.__table_args__

测试代码：

# saves the record with defaults
conn_type = FilteredConnectionType()
session.add(conn_type)
session.commit()

# raises an error: Check constraint failed
conn_type = FilteredConnectionType(filter_type=3)
session.add(conn_type)
session.commit()

注意事项：

FilteringMixin 扩展对象而不是 db.Model
我不是 100% 确定，但你可能不需要@declared_attr.cascading，而只需要@declared_attr
在FilteringMixin 中，我重命名了使用基于字符串的语法来定义约束。
完整的例子是here
sqlalchemy 文档参考 - declared_arrt 和 Mixin and Custom Base Classes

【讨论】：

谢谢。我理解您的代码为何有效，但它并不能完成一项重要任务。如果我将原始的FilteringMixin 混合到一些没有自己的__table_args__ 的模型中，我仍然会在该模型中得到正确的CheckContraint 而无需执行任何操作。随着您将其替换为 @classmethod，情况不再如此。
@Tomislav 这是一个很好的观点，我找到了另一个使用__table_args__ 的解决方案，约束是使用基于字符串的语法指定的。我更新了答案。
谢谢！这正是我一直在寻找的。最好能更好地了解declarative 的实际工作原理。据我了解，它需要已经初始化的类才能使用它们的属性。通过使用字符串表示，我们在 declarative 初始化之后推迟使用 CheckContraint 所需的类属性，这就是您的解决方案有效的原因。我已接受您的回答，想为您投票，但没有足够的代表。无论如何，再一次，这完全回答了我原来的问题。
@Tomislav 我无法解释为什么您的原始代码不起作用，但我觉得这只是一个粗糙的边缘，甚至可能是一个错误。查看文档中的示例，我希望CheckConstraint 的类对象语法应该可以工作。无论如何，我很高兴能够提供帮助。