结合使用带有Back_populates的关联对象时，SQLAlchemy抛出KeyError –文档中的示例不起作用 [英] SQLAlchemy throwing KeyError when using Association Objects with back_populates – example from documentation doesn't work

问题描述

SQLAlchemy很好地记录了如何在<$上使用关联对象c $ c> back_populates 。

SQLAlchemy nicely documents how to use Association Objects with back_populates.

但是，当从该文档中复制并粘贴示例时，会将子级添加到父代会抛出 KeyError ，如以下代码所示。从文档中复制模型类100％：

However, when copy-and-pasting the example from that documentation, adding children to a parent throws a KeyError as following code shows. The model classes are copied 100% from the documentation:

from sqlalchemy import Column, ForeignKey, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy.schema import MetaData

Base = declarative_base(metadata=MetaData())

class Association(Base):
    __tablename__ = 'association'
    left_id = Column(Integer, ForeignKey('left.id'), primary_key=True)
    right_id = Column(Integer, ForeignKey('right.id'), primary_key=True)
    extra_data = Column(String(50))
    child = relationship("Child", back_populates="parents")
    parent = relationship("Parent", back_populates="children")

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship("Association", back_populates="parent")

class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)
    parents = relationship("Association", back_populates="child")

parent = Parent(children=[Child()])

使用SQLAlchemy 1.2版运行该代码.11引发此异常：

Running that code with SQLAlchemy version 1.2.11 throws this exception:

lars$ venv/bin/python test.py
Traceback (most recent call last):
  File "test.py", line 26, in <module>
    parent = Parent(children=[Child()])
  File "<string>", line 4, in __init__
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/state.py", line 417, in _initialize_instance
    manager.dispatch.init_failure(self, args, kwargs)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 66, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 249, in reraise
    raise value
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/state.py", line 414, in _initialize_instance
    return manager.original_init(*mixed[1:], **kwargs)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/ext/declarative/base.py", line 737, in _declarative_constructor
    setattr(self, k, kwargs[k])
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 229, in __set__
    instance_dict(instance), value, None)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 1077, in set
    initiator=evt)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/collections.py", line 762, in bulk_replace
    appender(member, _sa_initiator=initiator)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/collections.py", line 1044, in append
    item = __set(self, item, _sa_initiator)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/collections.py", line 1016, in __set
    item = executor.fire_append_event(item, _sa_initiator)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/collections.py", line 680, in fire_append_event
    item, initiator)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 943, in fire_append_event
    state, value, initiator or self._append_token)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 1210, in emit_backref_from_collection_append_event
    child_impl = child_state.manager[key].impl
KeyError: 'parent'

我已将其作为 SQLAlchemy问题中的错误跟踪器。

推荐答案

tldr; ，也许有人可以同时向我指出一个可行的解决方案或解决方法？必须使用关联代理扩展和为关联对象创建一个自定义构造函数，该构造函数将子对象作为第一个（！）参数。

tldr; We have to use Association Proxy extensions and create a custom constructor for the association object which takes the child object as the first (!) parameter. See solution based on the example from the question below.

SQLAlchemy的文档实际上在下一段中指出，如果我们要直接添加<$ c，我们还没有完成。将$ c> Child 模型转换为 Parent 模型，同时跳过中介 Association 模型：

SQLAlchemy's documentation actually states in the next paragraph that we aren't done yet if we want to directly add Child models to Parent models while skipping the intermediary Association models:

以直接形式使用关联模式要求将
个子对象与关联实例相关联，然后再附加
给父母同样，从父级到子级的访问都通过关联对象进入
。

Working with the association pattern in its direct form requires that child objects are associated with an association instance before being appended to the parent; similarly, access from parent to child goes through the association object.

# create parent, append a child via association
p = Parent()
a = Association(extra_data="some data")
a.child = Child()
p.children.append(a)

要编写诸如问题中要求的简便代码，即 p.children = [Child（）] ，我们必须使用关联代理扩展。

To write convient code such as requested in the question, i.e. p.children = [Child()], we have to make use of the Association Proxy extension.

以下是使用关联代理扩展的解决方案，该扩展允许将孩子直接添加到父母中，而无需在他们之间明确创建关联：

Here is the solution using an Association Proxy extension which allows to add children to a parent "directly" without explicitly creating an association between both of them:

from sqlalchemy import Column, ForeignKey, Integer, String
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import backref, relationship
from sqlalchemy.schema import MetaData

Base = declarative_base(metadata=MetaData())

class Association(Base):
    __tablename__ = 'association'
    left_id = Column(Integer, ForeignKey('left.id'), primary_key=True)
    right_id = Column(Integer, ForeignKey('right.id'), primary_key=True)
    extra_data = Column(String(50))
    child = relationship("Child", back_populates="parents")
    parent = relationship("Parent", backref=backref("parent_children"))

    def __init__(self, child=None, parent=None):
        self.parent = parent
        self.child = child

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = association_proxy("parent_children", "child")

class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)
    parents = relationship("Association", back_populates="child")

p = Parent(children=[Child()])

不幸的是，我只弄清楚了如何使用 backref 而不是 back_populates ，这不是现代方法。

Unfortunately I only figured out how to use backref instead of back_populates which isn't the "modern" approach.

特别付款注意创建自定义的 __ init __ 方法，该方法将子级作为 first 参数。

Pay special attention to create a custom __init__ method which takes the child as the first argument.

这篇关于结合使用带有Back_populates的关联对象时，SQLAlchemy抛出KeyError –文档中的示例不起作用的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！