Best way to do polymorphism / tagged unions with an object in a field in an SQLModel?

[PhilippeGSK]

First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the SQLModel documentation, with the integrated search.
• I already searched in Google "How to X in SQLModel" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to SQLModel but to Pydantic.
• I already checked if it is not related to SQLModel but to SQLAlchemy.

Commit to Help

• I commit to help with one of those options 👆

Example Code

class ChannelUserMessage(BaseModel):
    type: Literal["user"]
    contents: str


class ChannelUserMessageWrite(BaseModel):
    type: Literal["user"]
    contents: str


class ChannelActionMessage(BaseModel):
    type: Literal["action"]
    contents: str
    character: CharacterPublic


class ChannelActionMessageWrite(BaseModel):
    type: Literal["action"]
    contents: str
    character_id: int


class ChannelDialogueMessage(BaseModel):
    type: Literal["dialogue"]
    contents: str
    character: CharacterPublic


class ChannelDialgueMessageWrite(BaseModel):
    type: Literal["dialogue"]
    contents: str
    character_id: int


ChannelMessageData = ChannelUserMessage | ChannelActionMessage | ChannelDialogueMessage
ChannelMessageDataAdapter = TypeAdapter[ChannelMessageData](ChannelMessageData)
ChannelMessageDataWrite = (
    ChannelUserMessageWrite | ChannelActionMessageWrite | ChannelDialgueMessageWrite
)


class ChannelMessage(SQLModel, table=True):
    id: int = Field(default=None, primary_key=True)
    channel_id: int = Field(foreign_key="channel.id")
    author_id: int = Field(foreign_key="user.id")
    author: User = Relationship()

    # Message data
    type: str = Field(default=None)
    contents: str | None = Field(default=None)
    character_id: int | None = Field(default=None, foreign_key="character.id")
    character: Character | None = Relationship()

    @computed_field
    @property
    def data(self) -> ChannelMessageData:
        return ChannelMessageDataAdapter.validate_python(self, from_attributes=True)

    @data.setter
    def data(self, val: ChannelMessageDataWrite):
        self.type = getattr(val, "type")
        self.contents = getattr(val, "contents", None)
        self.character_id = getattr(val, "character_id", None)


class ChannelMessagePublic(SQLModel):
    id: int
    channel_id: int

    author: UserPublic
    data: ChannelMessageData

Description

I'm making a small chat feature.

I'm trying to create a system where a field (data) in my SQLModel table (ChannelMessage) gets populated differently based on a type string. My current setup is in the code above. It works. I was wondering, though, if there were any cleaner and more automated ways to achieve the same result. I'm kind of new to the world of ORMs and things like that in general so I don't know about all the possibilities.

• I can create new "child" tables or reorganize my models if that will lead to a cleaner solution.
• I'd like to get a solution that doesn't involve me writing the same field names in so many places without type checking, because that's kind of a recipe for disaster for me personally.
• I'm following discord's model where a message response will contain a User object field instead of a user id. I will still have to decide on what will be an id. I'm using FastAPI so I can simply set response_model=list[ChannelMessagePublic] and it takes care of eliminating the fields I don't want it to include.

Operating System

Linux

Operating System Details

Debian GNU/Linux 12 (bookworm) x86_64

SQLModel Version

0.0.24

Python Version

Python 3.13.5

Additional Context

No response

[PhilippeGSK]

(I clicked one of the checkmarks to see what it would do I didn't think it would edit it)