SQLAlchemy error creating relationship between tables

I'm trying to make a simple database using SQLAlchemy from two tables: Users and Couple. The Couple table must contain a "couple" of users from the Users table.

For example:

Users:

Couple:

I tried to do this through “relationship” so that there was a one-to-many relationship, but it didn’t work. I ask for your help.

Here is an example of my models:

class Couple(Base):
    __tablename__ = 'couples'

    id = Column(Integer, primary_key=True, autoincrement=True, unique=True)
    first_user_ldap = relationship('Users', back_populates='Couple',
                                   lazy='dynamic')
    second_user_ldap = relationship('Users', back_populates='Couple',
                                    lazy='dynamic')


class Users(Base):
    __tablename__ = 'users'
    id = Column(String, primary_key=True, unique=True)
    user_name = Column(String)

I solved the problem. one of the likely reasons was “dirty imports”, which... captured models from other files.
Now:

from sqlalchemy import create_engine
from sqlalchemy.orm import Session, declarative_base
from sqlalchemy import Column, String, ForeignKey, Integer
from sqlalchemy.engine.url import URL

Models:

class Users(Base):
    __tablename__ = 'users'
    id = Column(String, primary_key=True, unique=True)
    user_name = Column(String)


class Couple(Base):
    __tablename__ = 'couples'

    id = Column(Integer, primary_key=True, autoincrement=True, unique=True)
    first_user_ldap = Column(String, ForeignKey('users.id'))
    second_user_ldap = Column(String, ForeignKey('users.id'))

DataSet:

    db.insert(Users(id=1, user_name='user1'))
    db.insert(Users(id=2, user_name='user2'))
    db.insert(Users(id=3, user_name='user3'))
    db.insert(Users(id=4, user_name='user4'))
    db.insert(Users(id=5, user_name='user5'))
    db.insert(Users(id=6, user_name='user6'))
    db.insert(Couple(first_user_ldap=1, second_user_ldap=2))
    db.insert(Couple(first_user_ldap=3, second_user_ldap=4))
    db.insert(Couple(first_user_ldap=5, second_user_ldap=6))