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

In a nutshell, do something like this:

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

Inside scripts it makes also sense to use the with-statement:

   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())
