Creating a family tree with fastAPI

Question

I have been trying to get fastAPI to return recursive family tree without any success.

class DBPerson(Base):
    __tablename__ = 'person'

    id = Column(Integer, primary_key=True)
    name = Column(String, index=True)
    ...

    father_id = Column(Integer, ForeignKey("person.id"), nullable=True)
    father = relationship("DBPerson", back_populates="children")
    mother_id = Column(Integer, ForeignKey("person.id"), nullable=True)
    mother = relationship("DBPerson", back_populates="children")

    children = relationship("DBPerson", back_populates=???)

And to create a family tree, I want both father and mother to return their father and mother...

class PersonBase(BaseModel):
    name: str
    id: int
    father: PersonBase | None = None
    mother: PersonBase | None = None


class PersonMinimal(PersonBase):
    class Config:
        orm_mode = True

Though doing it like this returns an error inside the PersonBase class with father and mother as the PersonBase class has not been defined yet.

I have seen examples of this where you have a "parent", but that does not tell me whether I have the mother or father, unless the object in the list would also return the gender... but I would rather do it this way if possible. Any help appreciated.

Also, if anyone knows a way to limit how many generations the recursive will go, that would also be appreciated.

I would be expecting the api to return the following:

{
    id: int
    name: str
    father: {
        id: int
        name: str
        father: { ... }
        mother: { ... }
    }
    mother: {
        id: int
        name: str
        father: { ... }
        mother: { ... }
    }
}

M.O. · Accepted Answer · 2024-05-31 08:38:11Z

0

You can either enter the type annotation as a string:

class PersonBase(BaseModel):
    name: str
    id: int
    father: "PersonBase | None" = None
    mother: "PersonBase | None" = None

or use from __future__ import annotations:

from __future__ import annotations

class PersonBase(BaseModel):
    name: str
    id: int
    father: PersonBase | None = None
    mother: PersonBase | None = None

See the pydantic docs for more information.

A word of warning however: if you're using PersonBase as a FastAPI response model, since both mother and father are also PersonBase, they will trigger recursive database calls until you've fetched the entire family tree. This might potentially be a costly operation if the tree gets deep, especially since the number of database roundtrips will grow exponentially.

answered May 31, 2024 at 8:38

M.O.

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