Insert a nested schema into a database with fastAPI? - python-3.x

Question

I have recently come to know about fastAPI and worked my way through the tutorial and other docs. Although fastAPI is pretty well documented, I couldn't find information about how to process a nested input when working with a database.
For testing, I wrote a very small family API with two models:
class Member(Base):
__tablename__ = 'members'
id = Column(Integer, primary_key=True, server_default=text("nextval('members_id_seq'::regclass)"))
name = Column(String(128), nullable=False)
age = Column(Integer, nullable=True)
family_id = Column(Integer, ForeignKey('families.id', deferrable=True, initially='DEFERRED'), nullable=False, index=True)
family = relationship("Family", back_populates="members")
class Family(Base):
__tablename__ = 'families'
id = Column(Integer, primary_key=True, server_default=text("nextval('families_id_seq'::regclass)"))
family_name = Column(String(128), nullable=False)
members = relationship("Member", back_populates="family")
and I created a Postgres database with two tables and the relations described here. With schema definitions and a crud file as in the fastAPI tutorial, I can create individual families and members and view them in a nested fashion with a get request. Here is the nested schema:
class Family(FamilyBase):
id: int
members: List[Member]
class Config:
orm_mode = True
So far, so good. Now, I would like to add a post view which accepts the nested structure as input and populates the database accordingly. The documentation at https://fastapi.tiangolo.com/tutorial/body-nested-models/ shows how to do this in principle, but it misses the database (i.e. crud) part.
As the input will not have id fields and obviously doesn't need to specify family_id, I have a MemberStub schema and the NestedFamilyCreate schema as follows:
class MemberStub(BaseModel):
name: str
age: int
class NestedFamilyCreate(BaseModel):
family_name: str
members: List[MemberStub]
In my routing routine families.py I have:
#app.post('/nested-families/', response_model=schemas.Family)
def create_family(family: schemas.NestedFamilyCreate, db: Session = Depends(get_db)):
# no check for previous existence as names can be duplicates
return crud.create_nested_family(db=db, family=family)
(the response_model points to the nested view of a family with all members including all ids; see above).
What I cannot figure out is how to write the crud.create_nested_family routine. Based on the simple create as in the tutorial, this looks like:
def create_nested_family(db: Session, family: schemas.NestedFamilyCreate):
# split information in family and members
members = family.members
core_family = None # ??? This is where I get stuck
db_family = models.Family(**family.dict()) # This fails
db.add(db_family)
db.commit()
db.refresh(db_family)
return db_family
So, I can extract the members and can loop through them, but I would first need to create a new db_family record which must not contain the members. Then, with db.refresh, I would get the new family_id back, which I could add to each record of members. But how can I do this? If I understand what is required here, I would need to achieve some mapping of my nested schema onto a plain schema for FamilyCreate (which works by itself) and a plain schema for MemberCreate (which also works by itself). But how can I do this?

Answer 1

I found a solution after re-reading about Pydantic models and their mapping to dict.
in crud.py:
def create_nested_family(db: Session, family: schemas.NestedFamilyCreate):
# split information in family and members
family_data = family.dict()
member_data = family_data.pop('members', None) # ToDo: handle error if no members
db_family = models.Family(**family_data)
db.add(db_family)
db.commit()
db.refresh(db_family)
# get family_id
family_id = db_family.id
# add members
for m in member_data:
m['family_id'] = family_id
db_member = models.Member(**m)
db.add(db_member)
db.commit()
db.refresh(db_member)
return db_family
Hope, this may be useful to someone else.