text: Title: How to accomplish Read/Write transactions with a one to many relationship
  Body: ### First Check
  
  - [X] I added a very descriptive title to this issue.
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  - [X] I searched the SQLModel documentation, with the integrated search.
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  - [X] I already read and followed all the tutorial in the docs and didn't find an answer.
  - [X] I already checked if it is not related to SQLModel but to [Pydantic](https://github.com/samuelcolvin/pydantic).
  - [X] I already checked if it is not related to SQLModel but to [SQLAlchemy](https://github.com/sqlalchemy/sqlalchemy).
  
  ### Commit to Help
  
  - [X] I commit to help with one of those options 👆
  
  ### Example Code
  
  ```python
  class User(SQLModel):
    __tablename__ = "users"
    id: Optional[str]
    cars: List[Car] = Relationship(sa_relationship=RelationshipProperty("Car", back_populates="user")
  
  class Car(SQLModel):
    ...
    user_id: str = Field(default=None, foreign_key="users.id")
    user: User = Relationship(sa_relationship=RelationshipProperty("User", back_populates="cars"))
    is_main_car: bool
  ```
  
  
  ### Description
  
  I have two tables that have a many to one relationship, such as the one described above. Any given user can only have a single car that `is_main_car`. Additionally, the first car a user gets must be the main car.
  
  I am trying to determine how the transactional semantics work with this relationship within a Session. If I read the `user`, and then use the `user.cars` field to determine if the user has 0 cars or already has a main car, can I rely on that condition still being true when I write my new main `Car` row to the `Cars` table (assuming it is all within a single Session)?
  
  ### Operating System
  
  macOS
  
  ### Operating System Details
  
  _No response_
  
  ### SQLModel Version
  
  0.0.4
  
  ### Python Version
  
  3.9.7
  
  ### Additional Context
  
  _No response_