bout the MVC pattern for architecting web apps and focus on how to integrate databases in Django projects.

Agenda

• MVC
  • how does this relate to what we've done so far
• The "M" in MVC
  • Adding a database to your Django project
  • Building models with Django's ORM
• Django shell
• Django's built-in admin interface

Django Shell

Run python manage.py shell to open an IPython shell with access to your app's database

• What is a database?
• Why do we need databases?

Components of a Database

• Tables: picture an Excel spreadsheet
• Columns: attributes, name, data type
• Row: Record, represents a set of data for each column
• Primary key:
  • id/pk
  • Unique
  • Can be an integer or UUID (349tu-hbe0984-5hww9e8y5h)
• Foreign key: Allows you to associate to another table. i.e. objectname_id
• ACID

Postgres

• How to run psql
  • On Linux, simply run psql anywhere in the terminal
  • On OS X, first make sure postgres is running (you should see the elephant in your toolbar). If it isn't open spotlight (cmd + space) and search for the Postgres application. If psql still doesn't work even though you see the elephant, try clicking on the elephant and telling it to start the Postgres server.
• Postgres commands:
  • \q - quits from Postgres
  • \l - lists all the databases in the system

Django's ORM

Useful Methods to Know

The complete Django db query interface can be found in the official documentation, but here are some highlights:

Creating

MyModel.create(name='someone', email='someone@somewhere.ca')

Reading

Retrieving a Single Object

MyModel.objects.get(pk=1)

MyModel.objects.get(name='something')

Retrieving Multiple Objects

MyModel.objects.all()

Conditions

Exact match:

MyModel.objects.filter(category='animals')

Greater than:

MyModel.objects.filter(price__gt=10)

Less than or equal to:

MyModel.objects.filter(price__lte=100)

Notice the double underscores: __ (called 'dunder' in Python lingo).

Counting Results

MyModel.objects.filter(category='animals').count()

Order

In ascending order:

MyModel.objects.order_by('price')

Or descending order:

MyModel.objects.order_by('-price')

Limit

The QuerySet returned can be treated like a list:

MyModel.objects.filter(category='animals')[0] MyModel.objects.filter(category='animals')[100:200]

Aggregate

These functions have a separate page in the docs

You can import extra aggregation functions:

from django.db.models import Avg, Sum, Max, Min

And use them like so:

MyModel.objects.aggregate(Avg('price'))

Deleting

example = MyModel.objects.get(pk=1)
example.delete()

Updating

example = MyModel.objects.get(pk=1)
example.name = 'Someone else'
example.save()

Querying the Database

Chain .query at the end of any Django query method to see what SQL is being composed and executed for you.