In FastAPI, how can I expand a model's output with a reverse URL lookup based on a field?

Question

Let's assume I have a simple file storage in the database. The SQLAlchemy model looks like this:

class Blob(Base):
    id = Column(Integer, primary_key=True)
    blob = deferred(Column(LargeBinary().with_variant(LONGBLOB, "mysql")))

The respective Pydantic model looks like this:

class BlobBase(sqlalchemy_to_pydantic(Blob, exclude=["blob"])):
    class Config:
        orm_mode = True

There is also a FastAPI route intented for fetching single files by ID:

@router.get("/blob/{blob_id}")
async def get_blob(blob_id: str):
   [...]

And a route for fetching a list of all files:

@router.get("/blobs", response_model=List[BlobBase])
async def get_blobs():
    return [BlobBase.from_orm(x) for x in db.session.query(Blob).all()]

Now, I would like to include the resolved get_blob URL in each entry in get_blobs. Naively, I suppose this should look like this:

class BlobBase(sqlalchemy_to_pydantic(Blob, exclude=["blob"])):
    class Config:
        orm_mode = True

    url: Optional[str]

    @validator("url")
    def make_url(cls, v, values):
        return request.url_for("get_blob", blob_id=values["id"])

However, I do not have access to a request or an app object in the validator, so that I cannot properly resolve the URL. NB: I do have access to the router object, which is an APIRouter for the current sub-URL, but get_blob in the actual application is in a different APIRouter, so I can't use that without jamming everything in a single file.

What is the correct way to solve this, i.e. include a resolved URL in model's output?