In this assignment you will temporarily step away from building the LocalLibrary applications and develop a data model from a file of un-normalized data and then build a script to load data in to that model.
The data is a simplified extraction of the UNESCO World Heritage Sites registry. The un-normalized data is provided as both a spreadsheet and a CSF file:
The columns in the data are as follows:
We will do this assignment within your library application but it will not have any user interface other than using the admin interface to verify that your application is working.
Make new application under your
cd ~/django_projects/locallibrary python3 manage.py startapp unesco
Also make a folder called
scripts and add an
__init__.py file to it. The
is needed in order to store Python objects in the
cd ~/django_projects/locallibrary mkdir scripts touch scripts/__init__.py
Make a copy of the
many_load.py from this folder into your
This is probably redundant, but make sure you are in your
django2 virtual environment and
workon django2 # If necessary pip3 install django_extensions
Add the following line to your
INSTALLED_APPS = [ 'django.contrib.admin', 'django.contrib.auth', 'django.contrib.contenttypes', ... 'django_extensions', # Add 'unesco.apps.UnescoConfig', # Add ]
You are to design a database model that represents this flat data across multiple tables using "third-normal form" - which basically means that columns that have vertical duplication, such as region:
category states region iso Cultural Afghanistan Asia and the Pacific af Cultural Afghanistan Asia and the Pacific af Cultural Albania Europe and North America al Cultural Albania Europe and North America al Cultural Algeria Arab States dz Mixed Algeria Arab States dz Cultural Algeria Arab States dz Cultural Algeria Arab States dz
You will make a Django model that describes the tables, one-to-many relationships, and foreign keys sufficient to represent this data efficiently with no vertical duplication. Numbers and dates do not have to have their own tables.
Name the first model
Site, use singular names for all of the table/model
names. Use the exact name of the column for the model field names and
foreign key names. Here is a subset of the
from django.db import models class Category(models.Model) : name = models.CharField(max_length=128) def __str__(self) : return self.name ... class Site(models.Model): name = models.CharField(max_length=128) year = models.IntegerField(null=True) category = models.ForeignKey(Category, on_delete=models.CASCADE) .... def __str__(self) : return self.name
All of the columns from the CSV data must be represented somewhere in the data model. There should be five models in your design, and four One-To-Many relationships and no Many-to-Many relationships.
Also add the models to
unesco/admin.py so you can view them in the administrator interface:
from django.contrib import admin # Register your models here. from unesco.models import Site, Category, ... admin.site.register(Site) admin.site.register(Category) ...
Once you have your model built, run
migrate to create
cd ~/django_projects/locallibrary python3 manage.py makemigrations python3 manage.py migrate
You can repeat the process of editing the
models.py file and re-running the migrations steps
until you get them right.
Django has a special
runscript capability that allows you to write a a Python program
to read and write the database using your Django models.
There is a simple example of how to write such a script in the
See the file
many_load.py for and example of how you look through a file,
insert model data and make foreign key connections. A key technique is in this bit of code:
p, created = Person.objects.get_or_create(email=row)
This code insures that there is a row in the Person table for the email address
that was just read
row. The email address may or may not already be in the table
from a previous line in the file. One way or another, by the end of this line
p contains a reference to a Person stored in the database that can be
used to fullfill a foreign key later in the code.
Note that the "p, created" is an example of Python function returning two values using a tuple.
m = Membership(role=r,person=p, course=c) m.save()
The line to make the
Membership row is the last thing that is done so all the
foreign key connections can be made.
Notice that the code empties the three tables out every time and freshly reloads all the data so the process can be run over and over.
Your data will be more complex than the sample, You will need to deal with situations
where an integer column like the
year will be empty. First, add
null=True to numeric columns
that can be empty in your
models.py. Then before inserting the
Site record, check the year to
see if it is a valid integer and if it is not a valid integer set it to
None which will become
NULL (or empty) in the data base when inserted:
try: y = int(row) except: y = None ... site = Site(name=row, description=row, year=y, ... ) site.save()
You will need to do this for each of the numeric fields that might be missing.
Place the CSV file in the
unesco folder and then run the script from the project folder (i.e.
manage.py file resides):
cd ~/django_projects/library workon django2 # if necessary python3 manage.py runscript many_load
It needs to be run this way so that lines like:
from unesco.models import Site, Iso, ....
You can check to see if your data was loaded properly in the Django Admin user interface.
You can also hand-check your data by running a few queries on your data before turning it in to make sure the data makes it into the right tables:
$ sqlite3 db.sqlite3 SQLite version 3.24.0 2018-06-04 14:10:15 Enter ".help" for usage hints. sqlite> SELECT count(id) FROM unesco_states; 163 sqlite> SELECT count(id) FROM unesco_site; 1044 sqlite> SELECT count(id) FROM unesco_states where name="India"; 1 sqlite> SELECT count(id) FROM unesco_site WHERE name="Hawaii Volcanoes National Park" AND year=1987 AND area_hectares = 87940.0; 1 sqlite> SELECT COUNT(*) FROM unesco_site JOIN unesco_iso ON iso_id=unesco_iso.id WHERE unesco_site.name="Maritime Greenwich" AND unesco_iso.name = "gb"; 1 sqlite>
We added this project to
locallibrary and added the models to the admin user interface so
you could look at them, but you might not want to see the
unesco data from this point forward.
Simply comment out the line in
locallibrary/locallibrary/settings.py as follows:
INSTALLED_APPS = [ 'django.contrib.admin', 'django.contrib.auth', 'django.contrib.contenttypes', ... 'django_extensions', # 'unesco.apps.UnescoConfig', # Comment out ]
And then restart your web application and verify that the unesco tables no longer show up in the administrator interface.