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Julio Cesar
Asked: 2020-05-10 14:44:54 +0800 CST 2020-05-10 14:44:54 +0800 CST

Why user_username? django

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I have a question when sending an object to me DetailView:

class ProfileDetailView(DetailView):
    model = Profile
    template_name = 'profiles/profile_detail.html'

    def get_object(self):
        return get_object_or_404(Profile, user__username=self.kwargs['username'])

This would be my model Profile:

class Profile(models.Model):
    user = models.OneToOneField(User, on_delete = models.CASCADE)
    avatar = models.ImageField(upload_to = custom_upload_to, null = True, blank = True)
    biography = models.TextField(null = True, blank = True)
    link = models.URLField(max_length = 200, null = True, blank = True)

My first question is, which field am I referring to in this part of the code?: ...,user_username=self...)Am I referring to usernamethe model Userfield or the usermodel field Profile?

My second question is why is there a reference to this field in this strange way?:user__username

And well, my last question is, how is it that get_objectthe dictionary receives the method self.kwargs? And how is it that the usernameuser's dictionary is found in that dictionary? I also saw that you could access the pkobject, through the self.kwargs, but I don't receive the pk, why is that?

Thanks in advance for your answers!

django

1 Answers

  1. Best Answer

    I'll help you answer and refer you to this page which I always reference for Django Class Based Views . Everything that I will answer you I get from there, it is a matter of interpreting the code and you will see the answer to each question.

    My first question is what field am I referring to in this part of the code?

    Something very incredible about the Django ORM is precisely how it handles "queries" (queries to the database) and one of its characteristics is the way in which it obtains and consults the tables related to the model. In this case, you have a named model Profilewith a field in its table that has a one-to-one relationship with table User. This logically means that a user has only one profile, and in the same way a profile belongs to only one user.

    To answer your question, you make two statements, and both are correct. If you are referring to the field usernameof Userand at the same time to the field userof Profile, why? because when you are going to query the database, normally you want to do a WHEREand filter the data you want, in this case you want to get a record from your table Profilebut the parameter you have to do the search in your database does not reference to table PROFILEif not USER. That is, you have is the usernameuser's. In Django this is possible.

    I explain further, you could consult a profile given a user, with the user's id, in this way:

    Profile.objects.get(user=1)

    You could just do it with an object User, like this:

    user = User.objects.first()
Profile.objects.get(user=user)

    But also, you could do it by querying the model fieldsUser

    Profile.objects.get(user__id=1)
# si te das cuenta esta consulta es igual a Profile.objects.get(user=1)

    What does that mean? That you can also consult for any other field that belongs to User:

    Profile.objects.get(user__username='Julio')
Profile.objects.get(user__email='[email protected]')
Profile.objects.get(user__password='123456')  # hasta por contraseña si quieres

    Django uses the expression __when you want to reference fields that belong to a related table. That is, as you can see in the example, neither username, nor emailnor passwordbelong to table Profile, so I must specify in the query which table they go to. leaving a structure like this

    "user" (la relacion) + "__" (Le decimos a Django que en la consulta haga un join) + "username" (un campo que solo puede pertenecer a "User")
"user__username" <== Este es el resultado

    In the end, this translates into saying this to Django: "Django, fetch me the profile record that, having a relation to the table, Userhas an User.usernameequal to the value that you specify"

    Another way to do this is the following:User.objects.get(username=self.kwargs['username']).profile

    So answering your question again, YES if you refer to userthe model field Profileand YES you also refer to usernamethe model field Userthanks to the fact that with __you can do a join.

    Why is there referencing this field in this weird way?

    Suddenly I can't get to why, let's just say the people who developed Django figured it out. But it is the way to do it, and it has worked well for everyone so far. As I have had the experience of working with several frameworks and ORMs such as Laravel, WordPress, Waterline, Sequelize and others, it seems to me that Django is the one that best manages queries and joins. It's just a matter of understanding it a little more so that you can see the potential that this has.

    How does get_objectthe dictionary receive the method self.kwargs?

    If you see the page that I recommend you read, you will see that it DetailViewis a class, so claiming that it get_objectreceives self.kwargsis not entirely valid, since it selfis the instance and kwargsit is a property of that instance. In fact, the place where that property is set is in the method as_view, if you read there is a line where it determines self.kwargs = kwargsand these kwargs come from the configuration of the urls, so it DetailViewdoes not fill it, but it receives it somewhere else of Django code that doesn't deserve to be explained, because as I already told you in an answer, It is not necessary to know how Django builds the request, that is Django's business and we must trust it, as long as we don't need any kind of metadata of the request.

    How is it that the usernameuser's dictionary is found in that dictionary? I also saw that you could access the pkobject, through the self.kwargs, but I don't get the pk, why is that?

    There is a question that I will not be able to answer you and that is the reason why he usernameis reaching you that way. The truth is that it will depend on the configuration of your urls, as I already answered in Duda with UpdateView in Django , the fact that you can receive parameters in the url will depend on your regular expression in the url. I invite you to read your previous question again and see how what you receive in the url works according to the groups in the regular expression. Most likely you don't have a pkbecause in the urls you don't have something like this: (?P<pk>\d+). But if you want, you can attach the code of your urls to help you better answer this point.

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