与 Association_proxy 的双向关系

Question

我有两个模型，Word 和 Sentence，它们具有双向多对多关系。为了存储更多信息，我有一个关联对象，WordInSentence。

class WordInSentence(Base):
    __tablename__ = "word_in_sentence"
    word_id = Column(Integer, ForeignKey('word.id'),
        primary_key=True)
    sentence_id = Column(Integer, ForeignKey('sentence.id'),
        primary_key=True)
    space_after = Column(String)
    tag = Column(String)
    position = Column(Integer)

    word = relationship("Word",
        backref=backref("word_sentences", lazy="dynamic"))
    sentence = relationship("Sentence",
        backref=backref("sentence_words", lazy="dynamic"))

class Sentence(Base):
    text = Column(Text, index = True)

    words = association_proxy("sentence_words", "word",
        creator=lambda word: WordInSentence(word=word))

class Word(Base):

    word = Column(String, index = True)
    sentences = association_proxy("word_sentences", "sentence",
        creator=lambda sent: WordInSentence(sentence=sent))

    def __repr__(self):
        return "<Word: " + str(self.word) + ">"

我希望能够做这样的事情：

w = Word()
s = Sentence()
w.sentences = [s]

但是，我收到如下错误：

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/plasma/project/venv/lib/python2.7/site-packages/sqlalchemy/ext/associationproxy.py", line 274, in __set__
    proxy.clear()
  File "/home/plasma/project/venv/lib/python2.7/site-packages/sqlalchemy/ext/associationproxy.py", line 629, in clear
    del self.col[0:len(self.col)]
TypeError: object of type 'AppenderBaseQuery' has no len()

我还注意到文档中的this example，但我不确定如何使其成为双向和列表。

Answer 1

您收到错误TypeError: object of type 'AppenderBaseQuery' has no len() 的原因是您的关系设置为lazy="dynamic"。如果您实际检查该对象，它只是一个 SQL 查询。这就是为什么你不能迭代它 - 它必须首先执行。

您可以通过使用适用于所有查询的标准函数来做到这一点 - 在对象上调用 filter(<conditions>)，或者如果您想要一切，则调用 all()。

如果您不希望每次访问时都执行动态查询的额外步骤，另一种选择（如果关系中的子项数量不大）是将lazy 设置更改为'select'。然后，这将在查询父对象的同时运行关联查询，这对于庞大的子数据库当然是不切实际的，但对于较小的子数据库来说并非不合理。然后就可以按照你的预期进行迭代了。

Answer 2

希望这段代码可以帮助到你。 我只是更改创建顺序并添加引擎和会话。 这是工作代码。

from sqlalchemy.engine import create_engine
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative.api import declarative_base
from sqlalchemy.orm import relationship, backref
from sqlalchemy.orm.session import Session
from sqlalchemy.sql.schema import Column, ForeignKey
from sqlalchemy.sql.sqltypes import Integer, String, Float, Date

Base = declarative_base()

class Sentence(Base):
    __tablename__ = 'sentence'

    id = Column(Integer, primary_key=True)
    text = Column(String, index = True)

    sentence_words = relationship('WordInSentence')
    words = association_proxy("sentence_words", "word")

class Word(Base):
    __tablename__ = 'word'

    id = Column(Integer, primary_key=True)
    word = Column(String, index = True)

    word_sentences = relationship('WordInSentence')
    sentences = association_proxy("word_sentences", "sentence")

    def __repr__(self):
        return "<Word: " + str(self.word) + ">"


class WordInSentence(Base):
    __tablename__ = "word_in_sentence"

    word_id = Column(Integer, ForeignKey(Word.id),
        primary_key=True)
    sentence_id = Column(Integer, ForeignKey(Sentence.id),
        primary_key=True)
    space_after = Column(String)
    tag = Column(String)
    position = Column(Integer)

    word = relationship(Word)
        #backref=backref("word_sentences", lazy="dynamic"))
    sentence = relationship(Sentence)
        #backref=backref("sentence_words", lazy="dynamic"))


engine = create_engine('sqlite://')
Base.metadata.create_all(engine)

session = Session(engine)

测试：

>>> 
>>> w = Word(word='something new')
>>> w.sentences = [Sentence(text='there is somethine new')]
>>> session.add(w)
>>> session.commit()
>>> session.query(WordInSentence).one().word_id
1
>>> session.query(Word).one().word
'something new'
>>> session.query(Sentence).one().text
'there is somethine new'
>>>