Configure Flask-SQLAlchemy

We want to add two imports to app.py:

app.py

from db import db

import models

The Flask app factory pattern

Up until now, we've been creating the app variable (which is the Flask app) directly in app.py.

With the app factory pattern, we write a function that returns app. That way we can pass configuration values to the function, so that we configure the app before getting it back.

This is especially useful for testing, but also if you want to do things like have staging and production apps.

To do the app factory, all we do is place all the app-creation code inside a function which must be called create_app().

app.py

from flask import Flask
from flask_smorest import Api

from db import db

import models

from resources.item import blp as ItemBlueprint
from resources.store import blp as StoreBlueprint

def create_app():
    app = Flask(__name__)
    app.config["PROPAGATE_EXCEPTIONS"] = True
    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/"
    api = Api(app)

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

    return app

Add Flask-SQLAlchemy code to the app factory

app.py

import os

from flask import Flask
from flask_smorest import Api

from db import db

import models

from resources.item import blp as ItemBlueprint
from resources.store import blp as StoreBlueprint

def create_app(db_url=None):
    app = Flask(__name__)
    app.config["PROPAGATE_EXCEPTIONS"] = True
    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 os.getenv("DATABASE_URL", "sqlite:///data.db")
    app.config["SQLALCHEMY_TRACK_MODIFICATIONS"] = False
    db.init_app(app)
    api = Api(app)

    with app.app_context():
        db.create_all()

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

    return app

We've done three things:

1. Added the db_url parameter. This lets us create an app with a certain database URL, or alternatively try to fetch the database URL from the environment variables. The default value will be a local SQLite file, if we don't pass a value ourselves and it isn't in the environment.
2. Added two SQLAlchemy values to app.config. One is the database URL (or URI), the other is a configuration option which improves performance.
3. When the app is created, tell SQLAlchemy to create all the database tables we need.

How does SQLAlchemy know what tables to create?

The line import models lets SQLAlchemy know what models exist in our application. Because they are db.Model instances, SQLAlchemy will look at their __tablename__ and defined db.Column attributes to create the tables.