Introduction into Contexts
**************************

If you are planning on using only one application you can largely skip
this chapter.  Just pass your application to the "SQLAlchemy"
constructor and you're usually set.  However if you want to use more
than one application or create the application dynamically in a
function you want to read on.

If you define your application in a function, but the "SQLAlchemy"
object globally, how does the latter learn about the former?  The
answer is the "init_app()" function:

   from flask import Flask
   from flask_sqlalchemy import SQLAlchemy

   db = SQLAlchemy()

   def create_app():
       app = Flask(__name__)
       db.init_app(app)
       return app

What it does is prepare the application to work with "SQLAlchemy".
However that does not now bind the "SQLAlchemy" object to your
application.  Why doesn't it do that? Because there might be more than
one application created.

So how does "SQLAlchemy" come to know about your application? You will
have to setup an application context.  If you are working inside a
Flask view function or a CLI command, that automatically happens.
However, if you are working inside the interactive shell, you will
have to do that yourself (see Creating an Application Context).

If you try to perform database operations outside an application
context, you will see the following error:

   No application found. Either work inside a view function or push an
   application context.

In a nutshell, do something like this:

>>> from yourapp import create_app
>>> app = create_app()
>>> app.app_context().push()

Alternatively, use the with-statement to take care of setup and
teardown:

   def my_function():
       with app.app_context():
           user = db.User(...)
           db.session.add(user)
           db.session.commit()

Some functions inside Flask-SQLAlchemy also accept optionally the
application to operate on:

>>> from yourapp import db, create_app
>>> db.create_all(app=create_app())
