SQLAlchemy:__ init __()需要1个位置参数,但是给出了2个(多对多)

Question

SQLAlchemy无疑是非常强大的,但是文档隐含地假定了许多先验知识和关系的主题,混合backref了新优选的back_populates()方法,我觉得很困惑.

以下模型设计几乎是文档中指南的精确镜像,该文档处理多对多关系的关联对象.您可以看到评论仍然与原始文章中的评论相同,我只更改了实际代码.

class MatchTeams(db.Model):
    match_id = db.Column(db.String, db.ForeignKey('match.id'), primary_key=True)
    team_id = db.Column(db.String, db.ForeignKey('team.id'), primary_key=True)
    team_score = db.Column(db.Integer, nullable="True")

    # bidirectional attribute/collection of "user"/"user_keywords"
    match = db.relationship("Match",
                            backref=db.backref("match_teams",
                                            cascade="all, delete-orphan")
                            )
    # reference to the "Keyword" object
    team = db.relationship("Team")


class Match(db.Model):
    id = db.Column(db.String, primary_key=True)

    # Many side of many to one with Round
    round_id = db.Column(db.Integer, ForeignKey('round.id'))
    round = db.relationship("Round", back_populates="matches")
    # Start of M2M

    # association proxy of "match_teams" collection
    # to "team" attribute
    teams = association_proxy('match_teams', 'team')

    def __repr__(self):
        return '<Match: %r>' % (self.id)


class Team(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String, nullable=False)
    goals_for = db.Column(db.Integer)
    goals_against = db.Column(db.Integer)
    wins = db.Column(db.Integer)
    losses = db.Column(db.Integer)
    points = db.Column(db.Integer)
    matches_played = db.Column(db.Integer)

    def __repr__(self):
        return '<Team %r with ID: %r>' % (self.name, self.id)

但是这个应该将团队实例find_liverpool与匹配实例find_match(两个样板对象)相关联的片段不起作用:

find_liverpool = Team.query.filter(Team.id==1).first()
print(find_liverpool)
find_match = Match.query.filter(Match.id=="123").first()
print(find_match)

find_match.teams.append(find_liverpool)

并输出以下内容:

Traceback (most recent call last):
File "/REDACT/temp.py", line 12, in <module>
find_match.teams.append(find_liverpool)
File "/REDACT/lib/python3.4/site-packages/sqlalchemy/ext/associationproxy.py", line 609, in append
item = self._create(value)
File "/REDACT/lib/python3.4/site-packages/sqlalchemy/ext/associationproxy.py", line 532, in _create
 return self.creator(value)
TypeError: __init__() takes 1 positional argument but 2 were given    
<Team 'Liverpool' with ID: 1>
<Match: '123'>

Answer 1

调用append试图创建一个新的实例中MatchTeams,从文件中可以看出.您在链接到的"简化关联对象"中也会注明到这一点:

如上所述,每项.keywords.append()操作相当于:

>>> user.user_keywords.append(UserKeyword(Keyword('its_heavy')))

因此你的

find_match.teams.append(find_liverpool)

相当于

find_match.match_teams.append(MatchTeams(find_liverpool))

由于MatchTeams没有明确定义__init__,它使用_default_constructor()as 构造函数(除非你重写它),它除了只接受关键字参数self,唯一的位置参数.

要解决此问题creator,请将工厂传递给您的关联代理:

class Match(db.Model):

    teams = association_proxy('match_teams', 'team',
                              creator=lambda team: MatchTeams(team=team))

或定义__init__上MatchTeams,以满足您的需求,例如:

class MatchTeams(db.Model):

    # Accepts as positional arguments as well
    def __init__(self, team=None, match=None):
        self.team = team
        self.match = match

或显式创建关联对象:

db.session.add(MatchTeams(match=find_match, team=find_liverpool))
# etc.