通过 sqlalchemy 关系添加值时如何避免插入重复条目？

Question

假设我们有两个多对多关系的表，如下所示：

class User(db.Model):
  __tablename__ = 'user'
  uid = db.Column(db.String(80), primary_key=True)
  languages = db.relationship('Language', lazy='dynamic',
                              secondary='user_language')

class UserLanguage(db.Model):
  __tablename__ = 'user_language'
  __tableargs__ = (db.UniqueConstraint('uid', 'lid', name='user_language_ff'),)

  id = db.Column(db.Integer, primary_key=True)
  uid = db.Column(db.String(80), db.ForeignKey('user.uid'))
  lid = db.Column(db.String(80), db.ForeignKey('language.lid'))

class Language(db.Model):
  lid = db.Column(db.String(80), primary_key=True)
  language_name = db.Column(db.String(30))

现在在 python shell 中：

In [4]: user = User.query.all()[0]

In [11]: user.languages = [Language('1', 'English')]

In [12]: db.session.commit()

In [13]: user2 = User.query.all()[1]

In [14]: user2.languages = [Language('1', 'English')]

In [15]: db.session.commit()

IntegrityError: (IntegrityError) column lid is not unique u'INSERT INTO language (lid, language_name) VALUES (?, ?)' ('1', 'English')

我怎样才能让关系知道它应该忽略重复项而不破坏 Language 表的唯一约束？当然，我可以分别插入每种语言，并事先检查表中是否已经存在该条目，但是这样就没有 sqlalchemy 关系提供的大部分好处了。

Answer 1

SQLAlchemy wiki 有一个 collection of examples，其中之一是您可能会如何 check uniqueness of instances。

这些例子有点复杂。基本上，创建一个类方法get_unique 作为备用构造函数，它将首先检查会话缓存，然后尝试查询现有实例，最后创建一个新实例。然后调用Language.get_unique(id, name) 而不是Language(id, name)。

我写了一个more detailed answer 来回应 OP 对另一个问题的赏金。

Answer 2

我建议阅读Association Proxy: Simplifying Association Objects。在这种情况下，您的代码将转换为如下内容：

# NEW: need this function to auto-generate the PK for newly created Language
# here using uuid, but could be any generator
def _newid():
    import uuid
    return str(uuid.uuid4())

def _language_find_or_create(language_name):
    language = Language.query.filter_by(language_name=language_name).first()
    return language or Language(language_name=language_name)


class User(Base):
  __tablename__ = 'user'
  uid = Column(String(80), primary_key=True)
  languages = relationship('Language', lazy='dynamic',
                              secondary='user_language')

  # proxy the 'language_name' attribute from the 'languages' relationship
  langs = association_proxy('languages', 'language_name',
            creator=_language_find_or_create,
            )

class UserLanguage(Base):
  __tablename__ = 'user_language'
  __tableargs__ = (UniqueConstraint('uid', 'lid', name='user_language_ff'),)

  id = Column(Integer, primary_key=True)
  uid = Column(String(80), ForeignKey('user.uid'))
  lid = Column(String(80), ForeignKey('language.lid'))

class Language(Base):
  __tablename__ = 'language'
  # NEW: added a *default* here; replace with your implementation
  lid = Column(String(80), primary_key=True, default=_newid)
  language_name = Column(String(30))

# test code
user = User(uid="user-1")
# NEW: add languages using association_proxy property
user.langs.append("English")
user.langs.append("Spanish")
session.add(user)
session.commit()

user2 = User(uid="user-2")
user2.langs.append("English") # this will not create a new Language row...
user2.langs.append("German")
session.add(user2)
session.commit()