SQLAlchemy ForeignKey and item creation

Question

I am trying to better understand SQL Alchemy to start using this ORM with Flask. I have two SQL tables. There is a many - one relationship between the two tables. I am trying to find a way by using SQL Alchemy to add an entry to the child table by using a foreing key but using a different field in the parent table.

│   .dockerignore
│   .env
│   .env-template
│   .gitignore
│   docker-compose.yml
│   Dockerfile
│   manage.py
│   README.md
│   requirements.txt
├───app
│   │   config.py
│   │   routes.py
│   │   __init__.py
│   │
│   ├───sites
│   │       controller.py
│   │       interface.py
│   │       model.py
│   │       schema.py
│   │       service.py
│   │       __init__.py
│   │
│   └───devices
│           controller.py
│           interface.py
│           model.py
│           schema.py
│           service.py
│           __init__.py
├───config
│       gunicorn.py

app --> sites --> model.py

from sqlalchemy import Integer, Column, String
from app import db  # noqa
from .interface import SiteInterface


class Site(db.Model):  # type: ignore
    """Site"""

    __tablename__ = "sites"

    site_db_id = Column(Integer(), primary_key=True, autoincrement=True, nullable=False)
    dc_site_id = Column(Integer(), unique=True)
    dc_name = Column(String(100))

app --> sites --> interface.py

from mypy_extensions import TypedDict


class SiteInterface(TypedDict, total=False):
    site_id: int
    dc_site_id: int
    dc_name: str

app --> sites --> schema.py

from marshmallow import fields, Schema


class SiteSchema(Schema):
    """Site schema"""

    siteId = fields.Number(attribute="site_db_id")
    dcsiteId = fields.Number(attribute="dc_site_id")
    dcname = fields.String(attribute="dc_name")

app --> devices --> model.py

from sqlalchemy import Integer, Column, String, ForeignKey
from sqlalchemy.orm import relationship
from app import db  # noqa
from .interface import DeviceInterface

class Device(db.Model):  # type: ignore
    """Device"""

    __tablename__ = "devices"

    device_id = Column(Integer(), primary_key=True, autoincrement=True, nullable=False)
    hostname = Column(String(50), nullable=False)
    ip = Column(String(15), unique=True)
    site_db_id = Column(Integer, ForeignKey('sites.site_db_id'))
    sites = relationship("Site")

app --> devices --> interface.py

from mypy_extensions import TypedDict


class DeviceInterface(TypedDict, total=False):
    deviceId: int
    hostname: str
    ip: str
    sitedbId: int

app --> devices --> schema.py

from marshmallow import fields, Schema


class DeviceSchema(Schema):
    """Device schema"""

    deviceId = fields.Number(attribute="device_id")
    hostname = fields.String(attribute="hostname")
    ip = fields.String(attribute="ip")
    sitedbId = fields.Number(attribute="site_db_id")

I need help to find the way to be able to add a new Device and specify the site_db_id using the dc_site_id

Something like this.

    @staticmethod
    def create(new_attrs: DeviceInterface) -> Device:
        new_device = Device(
            hostname=new_attrs["hostname"], ip=new_attrs["ip"],
            site_db_id=new_attrs["????"] *** select site_db_id FROM sites where dc_site_id=site_db_id ****
        )

        db.session.add(new_device)
        db.session.commit()

        return new_device

Thank you in advance for helping!!!

Answer 1

If I understand your requirement, you can use a two step approach: First select the Site using dc_site_id, then create the new Device with the Site found. Code could look for example like:

dc_site_id = new_attrs["dc_site_id "]
sitefound = Site.query.filter_by(dc_site_id = dc_site_id ).first_or_404()
new_device = Device(...,...., relatedsite = sitefound)

where relatedsite is the backref that you add in the Site class for your relationship.

devices= db.relationship('Device', backref='relatedsite', lazy=True)

This was this approach for one to many relationships that I found initially in the flask-sqlalchemy manual :

class Person(db.Model):
  id = db.Column(db.Integer, primary_key=True)
  name = db.Column(db.String(50), nullable=False)
  addresses = db.relationship('Address', backref='person', lazy=True)

class Address(db.Model):
  id = db.Column(db.Integer, primary_key=True)
  email = db.Column(db.String(120), nullable=False)
  person_id = db.Column(db.Integer, db.ForeignKey('person.id'), 
     nullable=False)