SQLModel: Many-to-many relationship data dropped when paginating

This is similar to https://github.com/uriyyo/fastapi-pagination/issues/294

However, when using SQLModel the relationship data is dropped. I’m always getting null for tags in my FastAPI response.

The queries seem fine. If I remove pagination, a simple unpaginated List of CustomerViews contains the tags. Maybe I’m doing something wrong with the model setup?

Excuse typos / weirdness. I had to pull this out of a large project and pare it down to make it readable.

# Relevant schema classes ----------------------------------------------

class CustomerViewToTagBase(SQLModel, TimeStampMixin):
    customer_view: int = Field(foreign_key="customer_view.id", primary_key=True)
    tag: int = Field(foreign_key="tag.id", primary_key=True)

class CustomerViewToTag(CustomerViewToTagBase, table=True):
    __tablename__ = "customer_view_to_tag"

class TagBase(SQLModel):
    name: str

class Tag(TagBase, table=True):
    __tablename__ = "tag"
    id: Optional[int] = Field(default=None, primary_key=True)
    customer_views: List["CustomerView"] = Relationship(back_populates="tags", link_model=CustomerViewToTag)

class CustomerViewBase(SQLModel):
    __tablename__ = "customer_view"
    name: Optional[str] = Field(default=None)

class CustomerView(CustomerViewBase, table=True):
    id: Optional[int] = Field(default=None, primary_key=True)
    tags: List["Tag"] = Relationship(back_populates="customer_views", link_model=CustomerViewToTag)

# For FastAPI
class CustomerViewWithTag(CustomerViewBase):
    id: int
    tags: Optional[List[Tag]]

    # I have no idea if this is necessary, I added it based on the linked issue above #294 
    class Config:
        orm_mode = True


# Router ------------------------------------------------------------------------

@router.get("/withtags", response_model=Page[CustomerViewWithTag], status_code=200)
async def get_customer_views_with_tags(
        db_session: AsyncSession = Depends(get_db_session),
        params: Params = Depends(),
        filter: Optional[str] = None
    ):
    async with db_session as db:
        customerviews_query = select(CustomerView).order_by(CustomerView.id).options(selectinload(CustomerView.tags))

        if filter is not None:
            customerviews_query = customerviews_query.filter(
                    CustomerView.name.ilike('%' + filter + '%')
            )

        # Switching the response_model to a List and the following code makes the relationship work properly
        # res = await db.execute(customerviews_query)
        # return res.scalars().all()
        return await paginate(session=db, query=customerviews_query, params=params)

[thebleucheese]

Hi @thebleucheese,

I will take a closer look

Thanks!

I’ll try to dig into it some more this weekend to see if I can find a fix unless you beat me to it.

PS - I love the project, it’s helped a lot with my first foray into using FastAPI

[uriyyo]

@thebleucheese Thanks for such a detailed investigation. I will close this ticket. Please, reopen it in case you still have some issues.