Accelerating Information Technology Innovation

Accelerating
Information Technology
Innovation
http://aiti.mit.edu
Rwanda Summer 2011
Django 01: Models
Models
•  Suppose we want to create a web
application to manage data about
thousands of movies
•  What information would we want to store?
–  Title
–  Rating (scale from 1 to 5)
–  Genre
–  Lead Actor
–  Supporting Actors
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Models
•  How would we think about storing this data?
–  Lecture 4: Dictionaries and lists are used to store
data in Python
•  Web applications use databases
–  Lots of options varying syntax
–  Each table represents a different model
–  Each column is a different attribute
•  Django: Common interface to almost all
database solutions (NoSQL and SQL)
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Models
•  Django’s database interface works with any
object of type django.db.models.Model
•  To create your own Model, use inheritance!
from django.db import models
class Movie(models.Model):
# attributes go here
Inherits from models.Model
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Models
•  Models have attributes: Fields
•  We create ‘instances’ of Model objects in a
different way (no __init__ function
necessary)
from django.db import models
class Movie(models.Model):
# attributes go here
self.title = models.CharField(max_length=100)
self.rating = models.IntegerField()
Field to store text (strings
and unicode objects)
Field to store the rating
the movie received
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Models
•  Some attributes indicate special relationships to other Model
objects
•  ForeignKey: OneToMany
•  ManyToManyField: Well, it’s a many-to-many field
from django.db import models
One movie can have many supporting
actors; one supporting actor can be in
many movies
class Movie(models.Model):
rating = models.IntegerField()
title = models.CharField(max_length=100)
genre = models.CharField()
lead_actor = models.ForeignKey(Actor,related_name=‘lead actor’)
support_actors = models.ManyToManyField(Actor,related_name=‘support’)
One lead actor who
could be the lead actor in other movies
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Checkpoint: Models
•  Build a django Model class for Actor
–  What does the class inherit from?
–  What attributes should the class have?
•  Build a django Model class for Award
–  What does the class inherit from?
–  What attributes should the class have?
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Checkpoint: Models
•  Actor Model class:
class Actor(models.Model):
name = models.CharField(max_length=100)
birth_date = models.DateField()
•  Attributes
•  name: a string- use a CharField
•  birth_date: a datetime.date- use a
DateField
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Checkpoint: Models
•  What type of field should we use for the title
of the Award?
•  CharField
•  What type of field should we use to denote
the winning actor?
•  ForeignKey (one actor, many awards)
•  What type of field should we use for the
nominees (each nominee is a Movie)?
•  ManyToManyField (each movie can be
nominated for many awards, each award
has many nominees)
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Checkpoint: Models
•  Award Model class:
•  What type of field should we use for the title of
the Award?
•  What type of field should we use to denote the
winning actor?
•  What type of field should we use for the
nominees (each nominee is a Movie)?
class Award(models.Model):
title = models.CharField(max_length=100)
sponsor = models.CharField(max_length=100)
year = models.DateField()
winning_actor = models.ForeignKey(Actor)
winning_movie = models.ForeignKey(Movie,related_name=‘winning movie’)
nominees = models.ManyToManyField(Movie,related_name=‘nominees’)
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Checkpoint: Models
•  Wait! How does Actor relate to the Movie
class that we just wrote?
•  We only need to specify the relation in one of
the models
•  Later: We can go “backwards” and get all
Movie objects with the same lead actor or
supporting actors
class Actor(models.Model):
name = models.CharField(max_length=100)
birth_date = models.DateField()
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Models
•  Inserting information into the database
–  Now: command line (manual)
–  Later: admin (Django GUI Interface)
–  Monday: forms (user-initiated)
•  But we need to actually create our
database first…
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Models:
•  Add the application with the relevant models to the
list of INSTALLED_APPS
•  Specify the path to your new database
•  Validate your Model classes
–  django-admin.py validate
•  Create or update the models in our project
–  python manage.py syncdb
•  Later: reset the database (clear all information)
–  python manage.py reset app_name
–  We’ll see the utility of this later on…
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Models: Python Shell
•  Use the Python shell to add information to
the database
–  python manage.py shell gives us a Djangoready Python shell
•  Let’s put information about ‘Akeelah and
the Bee’ in our database
–  Movie: Akeelah and the Bee
–  Actor: Keke Palmer
–  Award: Outstanding Actress (won by Keke)
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Models: Python Shell
from movie_app.models import Actor,Award,Movie
import datetime
Import the model
classes we made
keke = Actor(name=‘Keke Palmer’,birth_date=datetime.date(1993,8,26))
Looks like our usual initialization… but a little different
keke.save()
Saves a new entry in the Actor table of the database
How could we create a new entry for the movie ‘Akeelah and the Bee’
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Models: Python Shell
akeelah_bee = Movie(title=‘Akeelah and the Bee’,lead_actor=keke,
genre=‘Drama’,rating=4)
How do we store akeelah_award into our database?
akeelah_bee.save()
Create the award akeelah_bee
We need to stop and save akeelah_bee before setting our supporting actor
attributes
ManyToManyField: convenient, useful, and a little ‘weird’
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Models: Field Values
•  We know how to add entries into the
database with different attribute values
•  How do we restrict attribute values?
–  Set of choices
–  Validate using our own rules (consider the
max_length=100 rule that we have set on
some of the CharField objects)
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Models: Field Values
•  Suppose we wanted to add a gender attribute to
the Actor class
–  gender should be a character, either ‘M’ for ‘Male’ or
‘F’ for ‘Female’
•  The set of choices is a tuple of individual choices
•  Each choice is its own tuple
GENDER_CHOICES = ( (‘M’,’Male’),(‘F’,’Female’))
‘M’ and ‘F’ are the allowed field
values
‘Male’ and ‘Female’ are the human-readable
‘choices’ (what you will see in the admin
dropdown menu…see the next lecture)
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Models: Field Values
•  Restrict the movie rating to a number between 1
and 5
•  Use an adjective to describe the quality of the
movie
RATING_CHOICES = ((1,'Terrible'),(2,'Bearable'),(3,'Okay'),
(4,'Good'),(5,'Fantastic'))
class Movie(models.Model):
title = models.CharField(max_length=100)
# some attributes here
rating = models.IntegerField(choices=RATING_CHOICES)
Set the choices option to the RATING_CHOICES tuple
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Models: Access
•  Suppose our database has information on
three movies:
–  “Dreamgirls”
–  “Akeelah and the Bee”
–  “Something New”
•  All of these movies featured 2007 “Best
Actress” nominees (won by Keke Palmer
from “Akeelah and the Bee”)
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Models: Access
•  How do we access different subsets of these
movies?
•  QuerySet: iterable structure with zero, one, or
more table entries (model instances)
>>> Movie.objects.all()
[<Movie: Akeelah and the Bee>, <Movie: Dreamgirls>, <Movie: Something New>]
Returns a QuerySet with all model instances
>>> Movie.objects.filter(title="Dreamgirls")
[<Movie: Dreamgirls>]
Returns a QuerySet with model instances that have title ‘Dreamgirls’
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Models: Access
•  How do we access different subsets of these
movies?
•  QuerySet: iterable structure with zero, one, or
more table entries (model instances)
>>> Movie.objects.filter(rating__lt=5)
[<Movie: Akeelah and the Bee>, <Movie: Something New>]
Returns a QuerySet with model instances that have rating less than 5
>>> Movie.objects.filter(rating=5)
[<Movie: Dreamgirls>]
Returns a QuerySet with model instances that have rating equal to 5
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Models: Access
•  Access a particular Model instance:
–  Use the get command
•  Access to Model attributes:
–  Use the same .attribute syntax as in
Python classes
>>> dreamgirls = Movie.objects.get(title="Dreamgirls")
>>> print dreamgirls.lead_actor
Beyonce Knowles
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Backup Slides
•  References: IMDB.com
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Models: Using the Admin
•  admin is a builtin Django module
•  Simple GUI to create and modify the entries in your
database
models.py
class Actor(models.Model):
# some code
class Award(models.Model):
# some code
class Movie(models.Model):
# some code
admin.site.register(Actor)
admin.site.register(Award)
admin.site.register(Movie)
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