Here is how my tables are defined:

class User(Base):
   __tablename__ =  user 
   id: Mapped[int] = mapped_column(INTEGER, primary_key=True, init=False)
   username: Mapped[str] = mapped_column(VARCHAR, unique=True)

   purchases: Mapped[list[ Purchase ]] = relationship(back_populates=  user , init=False, cascade= all, delete , repr=False)

class Company(Base):
   __tablename__ =  company 
   id: Mapped[int] = mapped_column(INTEGER, primary_key=True, init=False)
   name: Mapped[str] = mapped_column(VARCHAR)
   
   products: Mapped[list[ Product ]] = relationship(back_populates= company , init=False, repr = False)


class Product(Base):
   __tablename__ =  product 
   id: Mapped[int] = mapped_column(INTEGER, primary_key=True, init=False)
   name: Mapped[str] = mapped_column(VARCHAR, index=True)

   company_id: Mapped[int] = mapped_column(ForeignKey( company.id ), init=False)

   company: Mapped[ Company ] = relationship(back_populates= products , repr = False)
   purchases: Mapped[list[ Purchase ]] = relationship(back_populates=  product , init=False, repr = False)


class Purchase(Base):
   __tablename__ =  purchase 
   id: Mapped[int] = mapped_column(INTEGER, primary_key=True, init=False)
   cost: Mapped[float] = mapped_column(NUMERIC(10, 2), index=True)

   user_id: Mapped[int] = mapped_column(ForeignKey( user.id ), init=False)
   product_id: Mapped[int] = mapped_column(ForeignKey( product.id ), init=False)

   user: Mapped[ User ] = relationship(back_populates= purchases , repr=False)
   product: Mapped[ Product ] = relationship(back_populates= purchases , repr=False)

This is my endpoint for getting user purchases:

@user_router.get( /get_user_purchases/{id} )
async def get_user_purchases(id: int, session: Session = Depends(get_session)):

      user = session.query(User).where(User.id == id).first()
      print(user.purchases) # works fine
      return user.purchases
      # RecursionError: maximum recursion depth exceeded while calling a Python object

Printing it to the terminal works fine, but why returning it from the endpoint results in maximum recursion depth exceeded error?