SQLAlchemy IntegrityError关系使用数据库中已经存在的对象

如何解决SQLAlchemy IntegrityError关系使用数据库中已经存在的对象

这是我的模型，它是用经典映射制作的，类A和B在相应地起作用

import sqlalchemy as sa
from sqlalchemy.orm import mapper,relationship

from domain.a import A
from domain.b import B
from app_extentions import Metadata

a_table = sa.Table(
    'a',Metadata,sa.Column('description',sa.String(30),primary_key=True),# I think this is important
    sa.Column('value_x',sa.Boolean()),)

b_table = sa.Table(
    'b',sa.Column('id',sa.BigInteger,primary_key=True,autoincrement=True),sa.String(50),sa.ForeignKey(a_table.c.description),nullable=False),sa.Column('value_y',sa.String(20),nullable=True),)

mapper(A,a_table)
mapper(B,b_table,properties={
    'rel': relationship(
        A,primaryjoin=(a_table.c.description == b_table.c.description)
    ),})

当我使用pytest执行此操作

obj1:A = retrieve_A_object() # A is already in the DB,I get it
obj2:B = create_B_object() # this is created Now,it is brand new
obj2.rel = obj1

session = get_session()
session.add(obj2)
session.commit()

sqlAlchemy引发错误

    def do_execute(self,cursor,statement,parameters,context=None):
>       cursor.execute(statement,parameters)
E       sqlalchemy.exc.IntegrityError: (psycopg2.errors.UniqueViolation) duplicate key value violates unique constraint "a_pkey"
E       DETAIL:  Key (description)=('MY DESCRIPTION') already exists.

我知道它已经在数据库中，我想保存B对象 我该如何解决？为什么会这样？