.. _quickstart:

Quickstart
==========

.. currentmodule:: flask_sqlalchemy

Flask-SQLAlchemy is fun to use, incredibly easy for basic applications, and
readily extends for larger applications.  For the complete guide, checkout
the API documentation on the :class:`SQLAlchemy` class.

A Minimal Application
---------------------

For the common case of having one Flask application all you have to do is
to create your Flask application, load the configuration of choice and
then create the :class:`SQLAlchemy` object by passing it the application.

Once created, that object then contains all the functions and helpers
from both :mod:`sqlalchemy` and :mod:`sqlalchemy.orm`.  Furthermore it
provides a class called ``Model`` that is a declarative base which can be
used to declare models::

    from flask import Flask
    from flask_sqlalchemy import SQLAlchemy

    app = Flask(__name__)
    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
    db = SQLAlchemy(app)


    class User(db.Model):
        id = db.Column(db.Integer, primary_key=True)
        username = db.Column(db.String(80), unique=True, nullable=False)
        email = db.Column(db.String(120), unique=True, nullable=False)

        def __repr__(self):
            return '<User %r>' % self.username

To create the initial database, just import the ``db`` object from an
interactive Python shell and run the
:meth:`SQLAlchemy.create_all` method to create the
tables and database::

    >>> from yourapplication import db
    >>> db.create_all()

Boom, and there is your database.  Now to create some users::

    >>> from yourapplication import User
    >>> admin = User(username='admin', email='admin@example.com')
    >>> guest = User(username='guest', email='guest@example.com')

But they are not yet in the database, so let's make sure they are::

    >>> db.session.add(admin)
    >>> db.session.add(guest)
    >>> db.session.commit()

Accessing the data in database is easy as a pie::

    >>> User.query.all()
    [<User u'admin'>, <User u'guest'>]
    >>> User.query.filter_by(username='admin').first()
    <User u'admin'>

Note how we never defined a ``__init__`` method on the ``User`` class?
That's because SQLAlchemy adds an implicit constructor to all model
classes which accepts keyword arguments for all its columns and
relationships.  If you decide to override the constructor for any
reason, make sure to keep accepting ``**kwargs`` and call the super
constructor with those ``**kwargs`` to preserve this behavior::

    class Foo(db.Model):
        # ...
        def __init__(self, **kwargs):
            super(Foo, self).__init__(**kwargs)
            # do custom stuff

Simple Relationships
--------------------

SQLAlchemy connects to relational databases and what relational databases
are really good at are relations.  As such, we shall have an example of an
application that uses two tables that have a relationship to each other::

    from datetime import datetime


    class Post(db.Model):
        id = db.Column(db.Integer, primary_key=True)
        title = db.Column(db.String(80), nullable=False)
        body = db.Column(db.Text, nullable=False)
        pub_date = db.Column(db.DateTime, nullable=False,
            default=datetime.utcnow)

        category_id = db.Column(db.Integer, db.ForeignKey('category.id'),
            nullable=False)
        category = db.relationship('Category',
            backref=db.backref('posts', lazy=True))

        def __repr__(self):
            return '<Post %r>' % self.title


    class Category(db.Model):
        id = db.Column(db.Integer, primary_key=True)
        name = db.Column(db.String(50), nullable=False)

        def __repr__(self):
            return '<Category %r>' % self.name

First let's create some objects::

    >>> py = Category(name='Python')
    >>> Post(title='Hello Python!', body='Python is pretty cool', category=py)
    >>> p = Post(title='Snakes', body='Ssssssss')
    >>> py.posts.append(p)
    >>> db.session.add(py)

As you can see, there is no need to add the ``Post`` objects to the
session. Since the ``Category`` is part of the session all objects
associated with it through relationships will be added too.  It does
not matter whether :meth:`db.session.add() <sqlalchemy.orm.session.Session.add>`
is called before or after creating these objects.  The association can
also be done on either side of the relationship - so a post can be
created with a category or it can be added to the list of posts of
the category.

Let's look at the posts. Accessing them will load them from the database
since the relationship is lazy-loaded, but you will probably not notice
the difference - loading a list is quite fast::

    >>> py.posts
    [<Post 'Hello Python!'>, <Post 'Snakes'>]

While lazy-loading a relationship is fast, it can easily become a major
bottleneck when you end up triggering extra queries in a loop for more
than a few objects.  For this case, SQLAlchemy lets you override the
loading strategy on the query level. If you wanted a single query to
load all categories and their posts, you could do it like this::

    >>> from sqlalchemy.orm import joinedload
    >>> query = Category.query.options(joinedload('posts'))
    >>> for category in query:
    ...     print category, category.posts
    <Category u'Python'> [<Post u'Hello Python!'>, <Post u'Snakes'>]


If you want to get a query object for that relationship, you can do so
using :meth:`~sqlalchemy.orm.query.Query.with_parent`.  Let's exclude
that post about Snakes for example::

    >>> Post.query.with_parent(py).filter(Post.title != 'Snakes').all()
    [<Post 'Hello Python!'>]


Road to Enlightenment
---------------------

The only things you need to know compared to plain SQLAlchemy are:

1.  :class:`SQLAlchemy` gives you access to the following things:

    -   all the functions and classes from :mod:`sqlalchemy` and
        :mod:`sqlalchemy.orm`
    -   a preconfigured scoped session called ``session``
    -   the :attr:`~SQLAlchemy.metadata`
    -   the :attr:`~SQLAlchemy.engine`
    -   a :meth:`SQLAlchemy.create_all` and :meth:`SQLAlchemy.drop_all`
        methods to create and drop tables according to the models.
    -   a :class:`Model` baseclass that is a configured declarative base.

2.  The :class:`Model` declarative base class behaves like a regular
    Python class but has a ``query`` attribute attached that can be used to
    query the model.  (:class:`Model` and :class:`BaseQuery`)

3.  You have to commit the session, but you don't have to remove it at
    the end of the request, Flask-SQLAlchemy does that for you.
