如何插入“答案”模型？

Question

标题可能不准确，但我不知道如何表达。

我有 3 个类：用户、问题、答案。简单的代码是：

Session = scoped_session(sessionmaker())
Base = declarative_base()

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)

    questions = relationship('Question', backref="user")
    answers = relationship('Answer', backref="user")

class Question(Base):
     __tablename__ = 'questions'
     id = Column(Integer, primary_key=True)
     user_id = Column(Integer, ForeignKey('users.id'))

     answers = relationship('Answer', backref="user")

class Answer(Base):
     __tablename__ = 'answers'
     user_id = Column(Integer, ForeignKey('users.id'))
     question_id = Column(Integer, ForeignKey('questions.id'))

     id = Column(Integer, primary_key=True)

现在，一位用户提出了一个问题，因此将创建一个答案：

user = get_user_from_session()
question = get_question(question_id)

# create answer
answer = Answer()
answer.user = user
answer.question = question

Session.add(answer)  # !!!
Session.commit()

希望将答案插入数据库，可惜报错：

 AttributeError: 'module' object has no attribute '_sa_instance_state'

有什么我错过的吗？如何解决？

---

更新

感谢@dhaffey，我已修正错别字。我重新创建了一个测试文件进行测试，发现没有再次发生错误，但是提交后answer.user_id和answer.question_id在数据库中为空。

这是我的代码，你可以直接运行。

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import *

engine = create_engine('sqlite:///test.sqlite', echo=True)
Session = scoped_session(sessionmaker())
Base = declarative_base()

Base.metadata.bind=engine

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    questions = relationship('Question')
    answers = relationship('Answer')

class Question(Base):
    __tablename__ = 'questions'
    id = Column(Integer, primary_key=True)
    user_id = Column(Integer, ForeignKey('users.id'))
    title = Column(String)
    answers = relationship('Answer')

class Answer(Base):
    __tablename__ = 'answers'
    user_id = Column(Integer, ForeignKey('users.id'))
    question_id = Column(Integer, ForeignKey('questions.id'))

    id = Column(Integer, primary_key=True)

Base.metadata.create_all()

user = User()
user.name = 'aaa'
Session.add(user)
Session.flush()

question = Question()
question.title = 'ttt'
question.user = user
Session.add(question)
Session.flush()

answer = Answer()
answer.user = user
answer.question = question

Session.add(answer)
Session.commit()

print answer.id # not None

found = Session.query(Answer).filter_by(id=answer.id).one()

print found.user.name # not None
print found.question.title # not None

# !!! It seems all models are saved correctly,
# but please open the test.sqlite database, (not querying by sqlahchemy)
# the question.user_id and answer.user_id and answer.question_id are null

Answer 1

你的类声明不会为我“编译”，所以我想知道你是否运行过这段代码，如果是的话，你使用的是哪个 SQLAlchemy 版本。线

user_id = Column(Integer, ForeignKey='users.id')

加注

sqlalchemy.exc.ArgumentError: Unknown arguments passed to Column: ['ForeignKey']

使用 SQLAlchemy 0.6.4。您正在尝试使用关键字参数声明外键，但正确的用法是构造一个 ForeignKey 对象并按位置传递它，如下所示：

user_id = Column(Integer, ForeignKey('users.id'))

固定外键后，您的示例按预期工作。

请注意，当相应的外键被适当声明时，您不需要在这些关系上显式提供 primaryjoin 参数 - SQLAlchemy 会推断出正确的连接。