将对象添加到SQLAlchemy关联对象时的KeyError

Question

我有两张桌子,tablet并且correspondent:

class Correspondent(db.Model, GlyphMixin):
    # PK column and tablename etc. come from the mixin
    name = db.Column(db.String(100), nullable=False, unique=True)
    # association proxy
    tablets = association_proxy('correspondent_tablets', 'tablet')

    def __init__(self, name, tablets=None):
        self.name = name
        if tablets:
            self.tablets = tablets


class Tablet(db.Model, GlyphMixin):
    # PK column and tablename etc. come from the mixin
    area = db.Column(db.String(100), nullable=False, unique=True)
    # association proxy
    correspondents = association_proxy('tablet_correspondents', 'correspondent')

    def __init__(self, area, correspondents=None):
        self.area = area
        if correspondents:
            self.correspondents = correspondents


class Tablet_Correspondent(db.Model):

    __tablename__ = "tablet_correspondent"
    tablet_id = db.Column("tablet_id",
        db.Integer(), db.ForeignKey("tablet.id"), primary_key=True)
    correspondent_id = db.Column("correspondent_id",
        db.Integer(), db.ForeignKey("correspondent.id"), primary_key=True)
    # relations
    tablet = db.relationship(
        "Tablet",
        backref="tablet_correspondents",
        cascade="all, delete-orphan",
        single_parent=True)
    correspondent = db.relationship(
        "Correspondent",
        backref="correspondent_tablets",
        cascade="all, delete-orphan",
        single_parent=True)

    def __init__(self, tablet=None, correspondent=None):
        self.tablet = tablet
        self.correspondent = correspondent

我可以将记录添加到tablet和correspondent,并且例如Tablet.query.first().correspondents只是返回一个空列表,正如您所期望的那样.如果我tablet_correspondent使用现有的平板电脑和对应的ID 手动在我的表中插入一行,则会再次按照您的预期填充列表.

但是,如果我尝试做的话

cor = Correspondent.query.first()
tab = Tablet.query.first()
tab.correspondents.append(cor)

我明白了:

KeyError: 'tablet_correspondents'

我很确定我在这里遗漏了一些相当基本的东西.

Answer 1

代码的问题在于.__init__方法.如果你是debug-watch/print()参数,你会注意到参数tablet实际上是一个实例Correspondent:

class Tablet_Correspondent(db.Model):
    def __init__(self, tablet=None, correspondent=None):
        print "in __init__: ", tablet, correspondent
        self.tablet = tablet
        self.correspondent = correspondent

这样做的原因是SA创建新值的方式.从文档创建新值:

当关联代理拦截列表append()事件(或集合add(),字典__setitem__()或标量赋值事件)时,它使用其构造函数实例化"中间"对象的新实例,将给定值作为单个参数传递.

在您调用的情况下tab.correspondents.append(cor),Tablet_Correspondent.__init__使用单个参数调用cor.

解？如果你只会被添加Correspondents到Tablet,然后就切换的参数__init__.实际上,完全删除第二个参数.
但是,如果您也将使用cor.tablets.append(tab),那么您需要明确地使用与上面链接的文档中所解释的creator参数association_proxy:

class Tablet(db.Model, GlyphMixin):
    # ...
    correspondents = association_proxy('tablet_correspondents', 'correspondent', creator=lambda cor: Tablet_Correspondent(correspondent=cor))

class Correspondent(db.Model, GlyphMixin):
    # ...
    tablets = association_proxy('correspondent_tablets', 'tablet', creator=lambda tab: Tablet_Correspondent(tablet=tab))