I have two objects, which I'm joining using an association proxy:

(doing this using Flask-sqlalchemy, so Base, engine etc are implicit)

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

However, I get a KeyError when I try to append an object (new or existing) to the Tablet.correspondents or Correspondent.tablets collection. I assume I've done something wrong when implementing the relationships and backrefs on the Tablet_Correspondent model – I was following the association proxy examples, but wanted it to be more symmetrical than the User.keywords example, which is where I'm going wrong, I suspect.

 

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