Using Flask-SQLAlchemy in Blueprint models without reference to the app [closed]

I believe the truest answer is that modular blueprints shouldn’t concern themselves directly with data access, but instead rely on the application providing a compatible implementation.

So given your example blueprint.

from flask import current_app, Blueprint, render_template

flat_pages = Blueprint('flat_pages', __name__, template_folder="templates")

@flat_pages.record
def record(state):
    db = state.app.config.get("flat_pages.db")

    if db is None:
        raise Exception("This blueprint expects you to provide "
                        "database access through flat_pages.db")

@flat_pages.route('/<page>')
def show(page):
    db = current_app.config["flat_pages.db"]
    page_object = db.find_page_by_name(page)
    return render_template('pages/{}.html'.format(page), page=page_object)

From this, there is nothing preventing you from providing a default implementation.

def setup_default_flat_pages_db(db):
    class Page(db.Model):
        name = db.Column(db.String(255), primary_key=True)
        title = db.Column(db.String(255))
        content = db.Column(db.String(255))

        def __init__(self, name, title, content):
            self.name = name
            self.title = title
            self.content = content

    class FlatPagesDBO(object):
        def find_page_by_name(self, name):
            return Page.query.filter_by(name=name).first()

    return FlatPagesDBO()

And in your configuration.

app.config["flat_pages.db"] = setup_default_flat_pages_db(db)

The above could be made cleaner by not relying in direct inheritance from db.Model and instead just use a vanilla declarative_base from sqlalchemy, but this should represent the gist of it.