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")

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	/stores/{id}/tags	Get a list of tags in a store.
✅ POST	/stores/{id}/tags	Create a new tag.
❌ POST	/items/{id}/tags/{id}	Link an item in a store with a tag from the same store.
❌ DELETE	/items/{id}/tags/{id}	Unlink a tag from an item.
✅ GET	/tags/{id}	Get information about a tag given its unique id.
❌ DELETE	/tags/{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("/stores/<string:store_id>/tags")
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("/tags/<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