I have been learning about how forms, and now ModelForms, work.
In a video by Max Goodridge, he redefines a field for one of his ModelFields in his ModelForm class. That is, he manually adds a field to his ModelForm class that could have been auto-generated by the ModelForm framework. From what I have read and understood thus far, that may be something to avoid. Though, that is not where my question lies.
I am wondering how redefining fields within a ModelForm class works. In the Django Docs, it is stated (with an example) that a ModelForm instance will have a form field for every model field specified. What happens then, when a form field is explicitly defined in a ModelForm instance? Are two fields generated or does ModelForm recognise that a field is already defined, thus not generating another one?
Furthermore, what exactly does adding an attribute to a ModelForm instance in the views do? For example, I have seen this:
form = ExampleForm(request.POST)
if form.is_valid():
post = form.save(commit=False)
post.user = request.user # herein lies my confusion
post.save()
What exactly is happening here? I have seen people do this and adding a timestamp as well, but I fail to understand exactly what it does. Presumably, the .save() method recognizes the attribute name 'user' and adds it to the database if the name corresponds with a Model-field name 'user'?
What happens when a form field is explicitly defined in a ModelForm instance?
How does adding an attribute with information for a model-field in a ModelForm instance work?
Thank you!
When you define a field at class level, the form will use that definition rather than create one from the model field. Far from being something to avoid, this is the correct thing to do if you want to completely customise a field.
Your second question is hard to understand. Save is not "recognising" anything. form.save() returns the instance of the model, on which you can set any field values as normal.
Related
When showing {{ user }} in a Django template, the default behavior is to show the username, i.e. user.username.
I'm changing this to show the user's initials instead, which are stored in a separate (OneToOneField) UserProfile model.
So in customsignup/models.py I've overridden the __unicode__ function successfully, with the desired result:
# __unicode__-function overridden.
def show_userprofile_initials(self):
return self.userprofile.initials
User.__unicode__ = show_userprofile_initials
But of course, the database is hit again because it needs to independently select the UserProfile model every time a user object is asked to show itself as a string. So even though this works, it escalates the number of database hits quite a bit.
So what I'd like to do, is to automatically use select_related('userprofile') whenever a User model is called from the database, seeing that I will essentially always want the profile when dealing with the user in the first place.
In more technical terms, I'm attempting to override the model manager of an existing model. So I'm in no control over the User model definition itself, since that's in an imported library.
So I've tried overriding the objects member of the User model in the same way that I overrode the __unicode__ function, like so:
# A model manager for automatically selecting the related userprofile-table
# when selecting from user-table.
class UserManager(models.Manager):
def get_queryset(self):
# Testing indicates that code here will NOT run.
return super(UserManager, self).get_queryset().select_related('userprofile')
User.objects = UserManager()
Is this supposed to work? If so, what am I getting wrong?
(I will mark an answer as correct if it can show that this is not supposed to work in the first place.)
A similar question I've found is here, but it's approached from the other end:
Automatically select related for OneToOne field
No, User.objects = MyManger() is not supposed to work. According to the docs, there are just two supported methods for extending the provided auth User model, either a profile model, as you are doing, or a proxy model, which probably doesn't fit your case. From the docs (emphasis added):
There are two ways to extend the default User model without substituting your own model. If the changes you need are purely behavioral, and don’t require any change to what is stored in the database, you can create a proxy model based on User. This allows for any of the features offered by proxy models including default ordering, custom managers, or custom model methods.
If you wish to store information related to User, you can use a OneToOneField to a model containing the fields for additional information. This one-to-one model is often called a profile model, as it might store non-auth related information about a site user.
As an alternative to extending the provided auth User model, you can provide your own custom User model. Then you will have complete control over its managers.
Instead, consider simply replacing {{ user }} with {{ user.profile.initials }}. Creating the OneToOne field on your profile model also creates a reverse accessor for instances of the related model. You can specify the reverse accessor name by the related_name keyword argument on the profile model field. For example...
from django.db import models
from django.contrib.auth.models import User
class UserProfile(models.Model)
user = models.OneToOneField('auth.User', related_name='profile')
initials = models.CharField(max_length=6)
some_user = User.objects.first()
# assuming there is already a profile related to this user
some_user.profile.initials = 'S.P.Y.'
You could also make a __str__ method for your profile model like
def __str__(self):
return self.initials
Then when you do {{ user.profile }} in a template, the initials will be shown.
I have a project in which I need to add a custom form field to a form (or formset) in which depending on the choice in the custom field selected, an integer in a database field is changed.
I can't seem to find any examples or prior questions which imply how to modify a database field via a custom field. I suspect it is done by overwriting the save() function in a ModelForm but cannot work out how.
Any advice on solving this problem would be greatly appreciated.
An Example:
My class has an integer field which is the field that needs updating.
class Employee(Model):
years = models.IntegerField()
However this field cannot be updated as it is, instead what is needed is a ChoiceField (I think) with different options that, depending on the selection, changes the field with a +1, -1, or reset to zero.
As previously mentioned this is where I imagine some work with the save() function needs to be done but am unsure.
As a side note in case this affects what can be done, eventually this ModelForm will need to be used in a formset so I can edit multiple objects on one page.
I am having an issue with the way Django class-based forms save a form. I am using a form.ModelForm for one of my models which has some many-to-many relationships.
In the model's save method I check the value of some of these relationships to modify other attributes:
class MyModel(models.Model):
def save(self, *args, **kwargs):
if self.m2m_relationship.exists():
self.some_attribute = False
super(MyModel, self).save(*args, **kwargs)
Even if I populated some data in the m2m relationship in my form, I self.m2m_relationship when saving the model and surprisingly it was an empty QuerySet. I eventually found out the following:
The form.save() method is called to save the form, it belongs to the BaseModelForm class. Then this method returns save_instance, a function in forms\models.py. This function defines a local function save_m2m() which saves many-to-many relationships in a form.
Here's the thing, check out the order save_instance chooses when saving and instance and m2m:
instance.save()
save_m2m()
Obviously the issue is here. The instance's save method is called first, that's why self.m2m_relationship was an empty QuerySet. It just doesn't exist yet.
What can I do about it? I can't just change the order in the save_instance function because it is part of Django and I might break something else.
Daniel's answer gives the reason for this behaviour, you won't be able to fix it.
But there is the m2m_changed signal that is sent whenever something changes about the m2m relationship, and maybe you can use that:
from django.db.models import signals
#signals.receiver(signals.m2m_changed, sender=MyModel.m2m_relationship.through)
def handle_m2m_changed(sender, instance, action, **kwargs):
if action == 'post_add':
# Do your check here
But note the docs say that instance "can be an instance of the sender, or of the class the ManyToManyField is related to".
I don't know how that works exactly, but you can try out which you get and then adapt the code.
But it would be impossible to do it any other way.
A many-to-many relationship is not a field on the instance, it is an entry in a linking table. There is no possible way to save that relationship before the instance itself exists, as it won't have an ID to enter into that linking table.
First of all I have tried to research my problem but have not been able to find what I need. My problem might be related to the design of my project (any help would be appreciated).
The problem I am facing is as follows:
I have a few models
I have a model that would be used specifically to create a ModelForm
In this model I have ForeignKey field that is represented by default as a select/option input widget in the ModelForm (for each the value attribute is the ForeignKey and text between the tags is the __str__() of the model the ForeignKey points to. The user sees the __str__() and value attribute of the option tag is submitted which is great).
So far so good but I want to replace the widget with an input text field so I can implement it as a search field.
Now when the user submits the form the string entered in the text input field is submitted and of course django doesn't like that since it expects a foreign key
I already can think of a few solutions to the problem and I am sure I can make it work but each of them feels like I would be violating some best practices. So my question is what should I do?
Do I exclude this particular field from the ModelForm and implement it as an input text field then after form submission make a query with it's value and then store the ForeignKey to the DB
Do I manipulate the data with JavaScript upon submission so that Django receives correct information
Can I clean this fields data with Django and transform it from string to FK?
Am I going the wrong way with this or there is a Django feature for this type of situation?
If anyone has the same problem here is the solution (to my problem at least):
I tried to use the clean_<fieldname> method to change the user entered string to database id. The method wasn't executing because the validation process was stopping earlier because of the difference between the form field and the widget. I redefined the form field to CharField so that step of the validation was working and then the clean_<fieldname> method executes without a problem.
I am programming an application in django, and I have a model where I defined some fields that are necessary to be filled. This way, when te user doesn't fill one of these fields, Django authomatically indicates to the user to fill it to create the specific object defined by the model.
But myquestion comes here: I want to reuse the same form to search objects defined by that model. And in this case, all the fields that before were necessary, now are OPTIONAL. But, as I have already defined the model so that the fields are necessary, django doesn´t let me define those fields as optional.
Is there any way to reuse that form where the fields are necessary, but making them OPTIONAL? Or I must create another different model or form in html? I know that creating another form manually in the html code the problem is solver, but I have curiosity to know if it can be reused.
Thank you so much!
You can programmatically change properties of a field within a form using its fields dictionary. So you could create a new form class that is derived from your current form class and in its __init__ set the required property of the fields you desired to be optional to be False like so:
self.fields['title'].required = False