SQLAlchemy 中的多对多关系

Question

我刚从MySQL转向SQLalchemy，必须说SQLalchemy比我想象的更难理解。目前，我在处理一个多对多关系时遇到了一些麻烦。我的模型中有用户和查询。一个用户可以有多个查询，而每个查询都提供一篇新文章以便阅读。我想要存储哪个用户在什么日期阅读了哪个查询。我的models.py看起来像这样：

class User(db.Model):

    id = db.Column(db.Integer, primary_key=True)
    read_dates = db.relationship("ReadIndex", backref="user")

    def __repr__(self):
        return '<User %r>' % (self.id)

class Queries(db.Model):

    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))

    def __repr__(self):
        return '<Queries %r>' % (self.id)


class ReadIndex(db.Model):

    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), primary_key=True)
    query_id = db.Column(db.Integer, db.ForeignKey('queries.id'), primary_key=True)
    read_datetime = db.Column(db.DateTime)
    read_query = db.relationship("Queries", backref="user_assocs")

    def __repr__(self):
        return '<ReadIndex>'

我有用户和查询。为了存储哪个用户在什么日期读取了哪个查询，我创建了ReadIndex类，并添加了一个名为read_datetime的额外字段，用于存储用户-查询关联的日期。然后我想要添加这样的关联：

user = models.User.query.filter_by(id=user_id).first()
query = models.Queries.query.filter_by(id=query_id).first()
a = models.ReadIndex(read_datetime=dt.utcnow())
a.read_query = query
user.read_dates.append(a)
db.session.commit()

其中query_id和user_id是选择我的对象的对应id。 如果我运行此操作，则会出现错误。

sqlalchemy.exc.IntegrityError
IntegrityError: (sqlite3.IntegrityError) NOT NULL constraint failed:
read_index.query_id [SQL: u'INSERT INTO read_index (user_id, read_datetime) 
VALUES (?, ?)'] [parameters: (1, '2015-07-29 20:55:50.898366')]

我不确定问题出在哪里？我似乎没有违反任何NOT NULL

编辑：我注意到我将query = None分配给了a.read_query。稍微更正了一下，错误就有所改变。

IntegrityError: (raised as a result of Query-invoked autoflush;  
consider using a session.no_autoflush block if this flush is occurring prematurely) 
(sqlite3.IntegrityError) NOT NULL constraint failed: read_index.user_id 
[SQL: u'INSERT INTO read_index (query_id, read_datetime) VALUES (?, ?)'] 
[parameters: (1, '2015-07-29 23:11:11.934038')]

以下是具体的发生情况：

>>> from app import app, db, models
>>> user = models.User.query.filter_by(id=1).first()
>>> user
<User u'Florian Beutler'>
>>> query = models.Queries.query.filter_by(id=1).first()
>>> query
<Queries 1>
>>> import datetime
>>> a = models.ReadIndex(read_datetime=datetime.datetime.utcnow())
>>> a.read_query = query
>>> user.read_dates.append(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-    packages/sqlalchemy/orm/attributes.py", line 237, in __get__
    return self.impl.get(instance_state(instance), dict_)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/attributes.py", line 578, in get
    value = self.callable_(state, passive)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/strategies.py", line 529, in _load_for_state
    return self._emit_lazyload(session, state, ident_key, passive)
  File "<string>", line 1, in <lambda>
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/strategies.py", line 599, in _emit_lazyload
    result = q.all()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2399, in all
    return list(self)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2515, in __iter__
    self.session._autoflush()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1292, in _autoflush
    util.raise_from_cause(e)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1282, in _autoflush
    self.flush()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 2004, in flush
    self._flush(objects)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 2122, in _flush
    transaction.rollback(_capture_exception=True)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 60, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 2086, in _flush
    flush_context.execute()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 373, in execute
    rec.execute(self)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 532, in execute
    uow
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/persistence.py", line 174, in save_obj
mapper, table, insert)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/persistence.py", line 781, in _emit_insert_statements
    execute(statement, params)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 914, in execute
    return meth(self, multiparams, params)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/sql/elements.py", line 323, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1010, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1146, in _execute_context
    context)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1341, in _handle_dbapi_exception
    exc_info
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1139, in _execute_context
    context)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 450, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.IntegrityError: (raised as a result of Query-invoked autoflush; consider using a session.no_autoflush block if this flush is occurring prematurely) (sqlite3.IntegrityError) NOT NULL constraint failed: read_index.user_id [SQL: u'INSERT INTO read_index (query_id, read_datetime) VALUES (?, ?)'] [parameters: (1, '2015-07-29 23:17:06.013485')]

约束？谢谢，卡尔。

- carl

Answer 1

.first() 方法会返回第一个结果，如果没有结果则返回 None。你确定你没有将 a.read_query 赋值为 None 吗？

- fishsticks

---

嗨，Fish，感谢你的回复。实际上我最初指定了None，但是更正并没有解决问题。我已经更新了上面的帖子以展示我的意思。 - carl

Answer 2

你之所以会遇到这个错误，是因为ReadIndex表的user_id和query_id字段都是None。因此，在提交时，ReadIndex对象尝试在其主键字段中没有值的情况下创建。所以，请执行以下操作：

user = models.User.query.filter_by(id=user_id).first()
query = models.Queries.query.filter_by(id=query_id).first()
a=models.ReadIndex(user_id=user.id,query_id=query.id,read_datetime=dt.utcnow())
db.session.add(a)
db.session.commit()

您无需手动添加任何关系。SQLAlchemy会自动为您完成。如果您想要跨检查，可以这样做。

for ud in user.read_dates:
    print(ud)

希望这能帮到你！

- Chandan Purohit