In a django form that has a manyToMany field - is there a database call to retrieve each object in the list when you're adding a new entry? Is this necessary/ wasteful?
For example:
class MyForm(ModelForm):
likes = forms.ModelMultipleChoiceField(queryset=Videos.objects.all())
....
the form is submitted with a list of project id's. within the clean() method likes becomes a list of Video objects. However you can do:
self.instance.likes.add() ...without ever having to get the objects.
...
is it wasteful that the objects are being retrieved first before updating the relationship? if not, why?
I would say that every time you save the form, it would be evaluating the queryset you passed it to check that each ID is within that queryset. if you wish to optimize the behavior, try subclassing ModelMultipleChoiceField and removing the checks/making them more streamlined.
Related
My django app connects to a sql server database. On my template I have an editable datatable/formset. Whenever a user edits a field and hits enter, I want only the edited field to be saved. As far as I know AJAX is the only approach to do this, especially if you want the value of the edited field to be the only one transfered to the server. Let's put this case aside for now and accept transferring all data of the formset at once.
Also, the formset is based on a sql server view that includes calculated values. Hence, executing formset.save() will raise an error. Instead I have to use the underlying table to save the modified value. What would be best practice to identify the changed field and save only this one?
Let's say the underlying table is costsperdiem that corresponds to a model in my django app and 'costs' is the name of the editable field I want to save then my approach is as follows:
check if the formset is valid
loop through the forms of the formset using changed_data to seek for the modified field
using a model instance to filter for the modified record
pass the modified value to the model
save the model
which looks like this in code:
formset = FormSet_CostsPerDiem(request.POST)
if formset.is_valid():
for f in formset.forms:
if 'costs' in f.changed_data:
val2save=f.cleaned_data['costs']
id=f.cleaned_data['id_costsperdiem']
rec2save=costsperdiem.objects.filter(pk=id)
rec2save.costs=val2save
rec2save.save()
What is best practice to do something like this?
In Django Admin for a Model I want all fields to be:
editable on creation
some of them on updating ( based on the instance fields values on creation).
For example:
2-1. If attribute a has a value, the fields corresponding to attributes c and b to be readonly
2-2. If attributes are empty after creation, should not be editable on updating
I know that for normal forms there is the Field disabled attribute.
I know I need to overwrite Admin form, but I don't have an idea, to know is created or update when form is initialized.
Usually I get the value using clean(), but here I need to get them on initialization in case of updates.
So it is like this:
You can create custom FORMS see here https://docs.djangoproject.com/en/1.8/ref/contrib/admin/#django.contrib.admin.ModelAdmin.form
After that you can add your logic of which form to use by overriding the get_form method. see here https://docs.djangoproject.com/en/1.8/ref/contrib/admin/#django.contrib.admin.ModelAdmin.get_form
However you need to make sure your DB will accept the partially submitted data. You can DROP NULL on the specific columns.
In my django app I have models set up similar to these models on the django site - Extra fields on many-to-many relationships. Further down the page, I read
The remove() method is disabled for similar reasons. However, the clear() method can be used to remove all many-to-many relationships for an instance:
If the remove method is disabled then how do I remove an object from a manytomany field? It says that I can use the clear method to remove everything but I only want to remove one specific element from the manytomany field.
You can remove the instance on the intermediary model.
From the example provided in djangoproject:
m_qs = Membership.objects.filter(person=person, group=group) #or some other logic to filter
try:
m = m_qs.get() #assuming queryset returns only 1 element
m.delete()
except:
pass #handle more gracefully
Can anyone explain this? It's really unintuitive that a form validation method would update the related model. I understand it doesn't commit to the database, but why even modify the model in memory? Why not do it with form.save()?
I believe the reason is this:
The first time you call is_valid() or
access the errors attribute of a
ModelForm has always triggered form
validation, but as of Django 1.2, it
will also trigger model validation.
This has the side-effect of cleaning
the model you pass to the ModelForm
constructor. For instance, calling
is_valid() on your form will convert
any date fields on your model to
actual date objects.
From here. Logically, this does actually make sense. Suppose I have a model like this:
class SomeModel(models.Model):
Somefield = models.CharField(unique=True)
Then as far as the modelform is concerned, validating the field TextInputField would work for any value. However, from the model's perspective, not any value will do - that value must be unique.
The validation mechanisms are described here. Basically, calling full_clean on a model causes that model to check its own fields as per the OO paradigm. So for this reason, the in memory image of the model can be updated and validating it will check it can actually be written to the database.
Thus it makes sense, because your guard:
if form.is_valid():
Really ought to ensure that:
form.save()
Can actually happen.
Here, I am a bit confused with forms in Django. I have information for the form(a poll i.e the poll question and options) coming from some db_table - table1 or say class1 in models. Now the vote from this poll is to be captured which is another model say class2. So, I am just getting confused with the whole flow of forms, here i think. How will the data be captured into the class2 table?
I was trying something like this.
def blah1()
get_data_from_db_table_1()
x = blah2Form()
render_to_response(blah.html,{...})
Forms have nothing to do with models in Django. They are just class meant to get informations from a dictionary (often request.POST) and check if each data linked to a key match a type and a format (e.g: is this a string of the form "bla#foo.tld").
You can ask django to create a form from a model, and in that case it will do its checking job, then if the data match, it will create a model, fill it and save it.
If a form is not created from a model, it will do nothing but checking. It will save nothing.
If it is created from a model, it will create a new instance of this particular model instance and save it.
If you want something more complicated, like, pre fill a form from various models or according to some conditions, or, say, you need to save several models according to the result of one form, you must do it manually.