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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Example: Understanding inheritance and relationships between model classes","articleBody":"### First Check\r\n\r\n- [X] I added a very descriptive title to this issue.\r\n- [X] I used the GitHub search to find a similar issue and didn't find it.\r\n- [X] I searched the SQLModel documentation, with the integrated search.\r\n- [X] I already searched in Google \"How to X in SQLModel\" and didn't find any information.\r\n- [X] I already read and followed all the tutorial in the docs and didn't find an answer.\r\n- [X] I already checked if it is not related to SQLModel but to [Pydantic](https://github.com/samuelcolvin/pydantic).\r\n- [X] I already checked if it is not related to SQLModel but to [SQLAlchemy](https://github.com/sqlalchemy/sqlalchemy).\r\n\r\n### Commit to Help\r\n\r\n- [X] I commit to help with one of those options 👆\r\n\r\n### Example Code\r\n\r\n```python\r\nclass Contract(SQLModel):\r\n    \"\"\"A contract defines the business conditions of a project\"\"\"\r\n    title: str = Field(description=\"Short description of the contract.\")\r\n    client: Client = Relationship(\r\n        back_populates=\"contracts\",\r\n    )\r\n    # Contract n:1 Client\r\n    client_id: Optional[int] = Field(\r\n        default=None,\r\n        foreign_key=\"client.id\",\r\n    )\r\n\r\n    currency: str  \r\n    term_of_payment: Optional[int] = Field(\r\n        description=\"How many days after receipt of invoice this invoice is due.\",\r\n        default=31,\r\n    )\r\n\r\n    \r\nclass TimeContract(Contract, table=True):\r\n    \"\"\"A time-based contract with a rate per time unit\"\"\"\r\n    id: Optional[int] = Field(default=None, primary_key=True)\r\n\r\n    rate: condecimal(decimal_places=2) = Field(\r\n        description=\"Rate of remuneration\",\r\n    )\r\n\r\n    unit: TimeUnit = Field(\r\n        description=\"Unit of time tracked. The rate applies to this unit.\",\r\n        sa_column=sqlalchemy.Column(sqlalchemy.Enum(TimeUnit)),\r\n        default=TimeUnit.hour,\r\n    )\r\n\r\nclass WorksContract(Contract, table=True):\r\n    \"\"\"A contract with a fixed price\"\"\"\r\n    id: Optional[int] = Field(default=None, primary_key=True)\r\n    price: condecimal(decimal_places=2) = Field(\r\n        description=\"Price of the contract\",\r\n    )\r\n    deliverable: str = Field(description=\"Description of the deliverable\")\r\n\r\nclass Client(SQLModel, table=True):\r\n    \"\"\"A client the freelancer has contracted with.\"\"\"\r\n\r\n    id: Optional[int] = Field(default=None, primary_key=True)\r\n    name: str\r\n    contracts: List[\"Contract\"] = Relationship(back_populates=\"client\")\r\n```\r\n\r\n\r\n### Description\r\n\r\nI would like to understand inheritance better using the following example:\r\n- There are two types of contracts, a time-based contract with a rate per time unit, and a works contract with a fixed price.\r\n- Every contract is related to a client.\r\n\r\nThe code example is my first attempt at implementing this. However, It is not yet correct:\r\n\r\n```\r\nInvalidRequestError: One or more mappers failed to initialize - can't proceed with initialization of other mappers. Triggering mapper: 'mapped class Client-\u003eclient'. Original exception was: When initializing mapper mapped class Client-\u003eclient, expression 'Contract' failed to locate a name ('Contract'). If this is a class name, consider adding this relationship() to the \u003cclass 'tuttle.model.Client'\u003e class after both dependent classes have been defined.\r\n```\r\n\r\nQuestions:\r\n\r\n- Every `Contract` is related to a client. That makes me think `client` should be a member of the `Contract` base class. However, I don't understand how to properly implement the relationship. Is it necessary to move `client` to the table classes (thereby duplicating it)?\r\n- When using inheritance from a model class, can I still `select` from `Contract` (rather than from the different contract types separately)?\r\n\r\n### Operating System\r\n\r\nmacOS, Other\r\n\r\n### Operating System Details\r\n\r\n_No response_\r\n\r\n### SQLModel Version\r\n\r\n0.0.8\r\n\r\n### Python Version\r\n\r\n3.10\r\n\r\n### Additional Context\r\n\r\n_No response_","author":{"url":"https://github.com/clstaudt","@type":"Person","name":"clstaudt"},"datePublished":"2022-11-05T08:48:54.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":13},"url":"https://github.com/488/sqlmodel/issues/488"}