Question

这是我的简单模型

  class Record(db.Model):
      __tablename__ = 'reg_records'
      request_id = db.Column(db.Integer, primary_key = True)
      title = db.Column(NVARCHAR())

  class Field(db.Model):
      __tablename__ = 'reg_fields'
      request_id = db.Column(db.Integer, primary_key= True) 
      name = db.Column(NVARCHAR(), primary_key = True)

现在我想将相关字段映射到记录。我正在遵循flask-sqlalchemy规范并获得此

  class Field(db.Model):
      __tablename__ = 'reg_fields'
      request_id = db.Column(db.Integer, ForeignKey('record.request_id'), primary_key= True) 
      name = db.Column(NVARCHAR(), primary_key = True)

  class Record(db.Model):
      __tablename__ = 'reg_records'
      request_id = db.Column(db.Integer, primary_key = True)
      title = db.Column(NVARCHAR())
      fields = db.relationship(Field, backref = 'record')

然而，当我尝试使用此模型时，我收到以下错误

  sqlalchemy.exc.NoForeignKeysError
  NoForeignKeysError: Could not determine join condition between parent/child tables on relationship Record.fields - there are no foreign keys linking these tables.  Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify a 'primaryjoin' expression.

将primaryjoin添加到关系声明中，如下所示

    fields = db.relationship(Field, backref = 'record', primaryjoin='Record.request_id==Field.request_id')

无法解决问题，而是提供新的异常

    sqlalchemy.exc.NoReferencedTableError
    NoReferencedTableError: Foreign key associated with column 'reg_fields.request_id' could not find table 'record' with which to generate a foreign key to target column 'request_id'

如果我用primaryjoin值中的表名替换类名，

    fields = db.relationship(Field, backref = 'record', primaryjoin='reg_records.request_id==reg_fields.request_id')

我得到另一个异常

   sqlalchemy.exc.InvalidRequestError
   InvalidRequestError: One or more mappers failed to initialize - can't proceed with initialization of other mappers.  Original exception was: 'Table' object has no attribute 'request_id'

总的来说，在flask-sqlalchemy中建立一对多关系的正确方法是什么。

Answer 1

目前还不是很清楚你要做什么。我假设您正在尝试在Record和Field之间建立一对多的关系（一个Record可以有零个或多个Fields）。我还假设您希望每个记录只有一个名为 name 的字段。以下模型设置了表格和关系，因为我相信您需要它们：

from sqlalchemy.schema import UniqueConstraint

class Record(db.Model):
    __tablename__ = 'reg_records'

    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(NVARCHAR())


class Field(db.Model):
    __tablename__ = 'reg_fields'
    __table_args__ = (
        UniqueConstraint('record_id', 'name', name='record_id_name_uix'),
    )

    id = db.Column(db.Integer, primary_key=True)
    record_id = db.Column(db.Integer, ForeignKey(Record.id))
    name = db.Column(NVARCHAR())

    record = db.relationship(Record, backref='fields')

您现在可以执行创建新记录和字段的操作：

record = Record(title='a record')
record.fields.append(Field(name='field1'))
record.fields.append(Field(name='field2'))
db.session.add(record)
db.session.commit()

但是，尝试为同一个Record创建两个具有相同名称的Fields，当您尝试提交它们时会得到IntegrityError：

record = Record(title='another record')
record.fields.append(Field(name='same_name'))
record.fields.append(Field(name='same_name'))
db.session.add(record)
db.session.commit()

最后，请尝试更加小心Python代码的格式，现在很难阅读。

Flask-SqlAlchemy关系错误

1 个答案: