title: Module has no mapped classes registered under the name '_sa_instance_state'
body: <p>I am using FastAPI SQLAlcamy, I made a project and all thing is well , When i am starting to do relations in models i faced alot of problems .The last of it
  ( Module 'request' has no mapped classes registered under the name '_sa_instance_state' ) .</p>
  <p>This is my models files. I will add my files in google drive any one can access it and contact with me : <a href="https://drive.google.com/drive/folders/1feB1whFTEkVTP6Emjud16Q1cu_NVw-JT?usp=sharing" rel="nofollow noreferrer">https://drive.google.com/drive/folders/1feB1whFTEkVTP6Emjud16Q1cu_NVw-JT?usp=sharing</a></p>
  <pre><code>from sqlalchemy import Table,Column,ForeignKey
  from sqlalchemy.sql.sqltypes import Integer,String,Text,Time
  from pydantic import BaseModel
  from config.db import Base,engine
  from sqlalchemy.orm import relationship
  
  
  class Request(Base):
      __tablename__ = 'requests'
      id = Column(Integer,primary_key = True,index=True)
      payment_id = Column(Integer,ForeignKey('payments.id'), index=True)
      slot_id = Column(Integer,ForeignKey('slots.id'), index=True)
      status = Column(Integer)
      started_at = Column(Time)
  
  
      qrcode = relationship(&quot;Qrcode&quot; ,back_populates = 'request_qrcode')
      payment = relationship(&quot;Payment&quot; ,back_populates = 'request_payment')
      slot = relationship(&quot;Slot&quot;,back_populates = 'request_slot')
  </code></pre>