One-to-many relationship between Tag and Store

Since we've already learned how to set up one-to-many relationships with SQLAlchemy when we looked at Items and Stores, let's go quickly in this section.

The SQLAlchemy models

models/tag.py

models/tag.py

from db import db


class TagModel(db.Model):
    __tablename__ = "tags"

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(80), unique=True, nullable=False)
    store_id = db.Column(db.String(), db.ForeignKey("stores.id"), nullable=False)

    store = db.relationship("StoreModel", back_populates="tags")

models/store.py

models/store.py

from db import db


class StoreModel(db.Model):
    __tablename__ = "stores"

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(80), unique=True, nullable=False)

    items = db.relationship("ItemModel", back_populates="store", lazy="dynamic")
    tags = db.relationship("TagModel", back_populates="store", lazy="dynamic")

Remember to import the TagModel in models/__init__.py so that it is then imported by app.py. Otherwise SQLAlchemy won't know about it, and it won't be able to create the tables.

The marshmallow schemas

These are the new schemas we'll add. Note that none of the tag schemas have any notion of "items". We'll add those to the schemas when we construct the many-to-many relationship.

In the StoreSchema we add a new list field for the nested PlainTagSchema, just as it has with PlainItemSchema.

schemas.py

class PlainTagSchema(Schema):
    id = fields.Int(dump_only=True)
    name = fields.Str()


class StoreSchema(PlainStoreSchema):
    items = fields.List(fields.Nested(PlainItemSchema()), dump_only=True)
    tags = fields.List(fields.Nested(PlainTagSchema()), dump_only=True)


class TagSchema(PlainTagSchema):
    store_id = fields.Int(load_only=True)
    store = fields.Nested(PlainStoreSchema(), dump_only=True)

The API endpoints

Let's add the Tag endpoints that aren't related to Items:

Method	Endpoint	Description
✅ GET	/store/{id}/tag	Get a list of tags in a store.
✅ POST	/store/{id}/tag	Create a new tag.
❌ POST	/item/{id}/tag/{id}	Link an item in a store with a tag from the same store.
❌ DELETE	/item/{id}/tag/{id}	Unlink a tag from an item.
✅ GET	/tag/{id}	Get information about a tag given its unique id.
❌ DELETE	/tag/{id}	Delete a tag, which must have no associated items.

Here's the code we need to write to add these endpoints:

resources/tag.py

from flask.views import MethodView
from flask_smorest import Blueprint, abort
from sqlalchemy.exc import SQLAlchemyError

from db import db
from models import TagModel, StoreModel
from schemas import TagSchema

blp = Blueprint("Tags", "tags", description="Operations on tags")


@blp.route("/store/<string:store_id>/tag")
class TagsInStore(MethodView):
    @blp.response(200, TagSchema(many=True))
    def get(self, store_id):
        store = StoreModel.query.get_or_404(store_id)

        return store.tags.all()

    @blp.arguments(TagSchema)
    @blp.response(201, TagSchema)
    def post(self, tag_data, store_id):
        if TagModel.query.filter(TagModel.store_id == store_id).first():
            abort(400, message="A tag with that name already exists in that store.")

        tag = TagModel(**tag_data, store_id=store_id)

        try:
            db.session.add(tag)
            db.session.commit()
        except SQLAlchemyError as e:
            abort(
                500,
                message=str(e),
            )

        return tag


@blp.route("/tag/<string:tag_id>")
class Tag(MethodView):
    @blp.response(200, TagSchema)
    def get(self, tag_id):
        tag = TagModel.query.get_or_404(tag_id)
        return tag

Register the Tag blueprint in app.py

Finally, we need to remember to import the blueprint and register it!

app.py

from flask import Flask
from flask_smorest import Api

import models

from db import db
from resources.item import blp as ItemBlueprint
from resources.store import blp as StoreBlueprint
from resources.tag import blp as TagBlueprint


def create_app(db_url=None):
    app = Flask(__name__)
    app.config["API_TITLE"] = "Stores REST API"
    app.config["API_VERSION"] = "v1"
    app.config["OPENAPI_VERSION"] = "3.0.3"
    app.config["OPENAPI_URL_PREFIX"] = "/"
    app.config["OPENAPI_SWAGGER_UI_PATH"] = "/swagger-ui"
    app.config[
        "OPENAPI_SWAGGER_UI_URL"
    ] = "https://cdn.jsdelivr.net/npm/swagger-ui-dist/"
    app.config["SQLALCHEMY_DATABASE_URI"] = db_url or "sqlite:///data.db"
    app.config["SQLALCHEMY_TRACK_MODIFICATIONS"] = False
    app.config["PROPAGATE_EXCEPTIONS"] = True
    db.init_app(app)
    api = Api(app)

    @app.before_first_request
    def create_tables():
        db.create_all()

    api.register_blueprint(ItemBlueprint)
    api.register_blueprint(StoreBlueprint)
    api.register_blueprint(TagBlueprint)

    return app