CODEWITHSUNDEEP
django
fransua
fransua

Reputation: 533

Django / How to get the primary key from a specific field?

Models.py

class scenes(models.Model):
    name = models.CharField('Event name', max_length=120)
    record_date = models.DateTimeField('Event date')

Let's say I have recorded a scenes with name="world"

In views.py, how can I query the pk from the name field ?

from .models import scenes
 scene = scenes.objects.get('what should I put here to get the pk associated with name World ?')

When I entered :

scene = scenes.objects.get(name='world').pk

I got an error :

UnboundLocalError: local variable 'scenes' referenced before assignment

Upvotes: 2

Views: 2239

Answers (2)

Carlos Osuna
Carlos Osuna

Reputation: 181

see this: The pk lookup shortcut.

The django documentation declares that primary keys have an equivalent, in this case it is pk. So if you declared a primary key in your model (let's say we called it code) you could access it as code or pk. This is useful both to apply filters (like the link above) or to obtain a specific attribute.

Now your question is how to obtain the associated primary key from the name of the event, I will show you below some solutions with consequences to how you declared the model.

A. Using scenes.objects.get(): If you use this method you must take into consideration two things:

  1. That the search exists, it not exit, it raises Model.DoesNotExist exception.
  2. That the search return only 1 object,it finds more than one object, it raises a Model.MultipleObjectsReturned exception:

please see the Queryset get() method

so if we ignore the second thing, the block of code is like this

# we have 1 object with the name `world`

try:
    scene = scenes.objects.get(name="world")
    scene_pk = scene.pk
except scenes.DoesNotExist: 
    print("The Scene does not Exists")

but the second you should use Queryset filter() method with other methods like first() or last()

So I recommend you re-make the Model like this:

class scenes(models.Model):
    name = models.CharField('Event name',unique=True, max_length=120)
    record_date = models.DateTimeField('Event date')

or using a SlugField like pk or unique field if you don't wanna change the id as primary key

class scenes(models.Model):
    name = models.CharField('Event name', max_length=120)
    slug = models.SlugField(unique=True)
    record_date = models.DateTimeField('Event date')

the slugfield is ideal if you wish to use the name of the event in the URL.

Upvotes: 1

mtzd
mtzd

Reputation: 789

The easy way to go with that would be to just:

# views.py

from .models import scenes

scene = scenes.objects.get(name="world")
scene_id = scene.pk # This contains the pk of the scene object

Django will try to fetch a unique object with "world" as a value for the name field. With your current code this has an issue:

  • Your name field is not unique so your DB may contain different scenes objects with the "world" value for the name field. This will lead to problems when calling get. To solve that you could add unique=True when defining the field:
# models.py

class scenes(models.Model):
    name = models.CharField('Event name', max_length=120, unique=True)
    record_date = models.DateTimeField('Event date')

This will ensure that your DB won't contain objects with the same name.

Note also that if there's no object with name value equal to "world" you'll get an error too. Depending on the context you are using this, you should consider get_object_or_404 or get_or_create

Upvotes: 2

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