H a n d s o n, p r o j e c t b a s e d

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Chapter 18
When you click Save, you’ll be brought back to the main admin page 
for entries. Here, you’ll see the benefit of using 
text[:50]
as the string rep-
resentation for each entry; it’s much easier to work with multiple entries in 
the admin interface if you see only the first part of an entry rather than the 
entire text of each entry. 
Make a second entry for Chess and one entry for Rock Climbing so we 
have some initial data. Here’s a second entry for Chess:
In the opening phase of the game, it’s important to bring out 
your bishops and knights. These pieces are powerful and maneu-
verable enough to play a significant role in the beginning moves 
of a game.
And here’s a first entry for Rock Climbing:
One of the most important concepts in climbing is to keep 
your weight on your feet as much as possible. There’s a myth that 
climbers can hang all day on their arms. In reality, good climb-
ers have practiced specific ways of keeping their weight over their 
feet whenever possible.
These three entries will give us something to work with as we continue 
to develop Learning Log.
The Django Shell
With some data entered, we can examine that data programmatically 
through an interactive terminal session. This interactive environment is 
called the Django shell, and it’s a great environment for testing and trouble-
shooting your project. Here’s an example of an interactive shell session:
(ll_env)learning_log$ python manage.py shell
u
>>> from learning_logs.models import Topic
>>> Topic.objects.all()
, ]>
The command 
python manage.py shell
, run in an active virtual environ-
ment, launches a Python interpreter that you can use to explore the data 
stored in your project’s database. Here, we import the model 
Topic
from the 
learning_logs.models
module u. We then use the method 
Topic.objects.all()
to get all the instances of the model 
Topic
; the list that’s returned is called a 
queryset.
We can loop over a queryset just as we’d loop over a list. Here’s how you 
can see the ID that’s been assigned to each topic object:
>>> topics = Topic.objects.all()
>>> for topic in topics:
... print(topic.id, topic)

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Getting Started with Django
393
...
1 Chess
2 Rock Climbing
We store the queryset in 
topics
, and then print each topic’s 
id
attribute 
and the string representation of each topic. We can see that Chess has an 
ID of 1, and Rock Climbing has an ID of 2.
If you know the ID of a particular object, you can use the method 
Topic 
.objects.get()
to retrieve that object and examine any attribute the object 
has. Let’s look at the 
text
and 
date_added
values for Chess:
>>> t = Topic.objects.get(id=1)
>>> t.text
'Chess'
>>> t.date_added
datetime.datetime(2019, 2, 19, 1, 55, 31, 98500, tzinfo=)
We can also look at the entries related to a certain topic. Earlier we 
defined the 
topic
attribute for the 
Entry
model. This was a 
ForeignKey
, a con-
nection between each entry and a topic. Django can use this connection to 
get every entry related to a certain topic, like this:
u
>>> t.entry_set.all()
, 
In the opening phase of the game, it's important t...>]>
To get data through a foreign key relationship, you use the lowercase 
name of the related model followed by an underscore and the word 
set
u. 
For example, say you have the models 
Pizza
and 
Topping
, and 
Topping
is related 
to 
Pizza
through a foreign key. If your object is called 
my_pizza
, representing 
a single pizza, you can get all of the pizza’s toppings using the code 
my_pizza 
.topping_set.all()
.
We’ll use this kind of syntax when we begin to code the pages users 
can request. The shell is very useful for making sure your code retrieves the 
data you want it to. If your code works as you expect it to in the shell, you 
can expect it to work properly in the files within your project. If your code 
generates errors or doesn’t retrieve the data you expect it to, it’s much eas-
ier to troubleshoot your code in the simple shell environment than within 
the files that generate web pages. We won’t refer to the shell much, but you 
should continue using it to practice working with Django’s syntax for access-
ing the data stored in the project.
n o t e
 
Each time you modify your models, you’ll need to restart the shell to see the effects of 
those changes. To exit a shell session, press 
ctrl
-D; on Windows, press 
ctrl
-Z and 
then press 
enter
.

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