Flask-SqlAlchemy 关系错误

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 规范并得到这个

  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')

但是，当我尝试使用此模型时，出现以下错误

  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 添加到如下关系声明中

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

没有解决问题，而是给出了新的异常

    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')

我又遇到了一个例外

   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

目前还不是很清楚您要做什么。我假设您正在尝试在记录和字段之间建立一对多的关系（一个记录可以有零个或多个字段）。我还假设您希望每条记录只有一个名为 name 的字段。以下模型根据您的需要设置表和关系：

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等操作：

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，尝试为同一条记录创建两个同名的字段，当您尝试提交它们时会得到一个 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，请尽量注意 Python 代码的格式，因为它很难阅读。