I have a foreign key relationship in my Django (v3) models:
class Example(models.Model):
title = models.CharField(max_length=200) # this is irrelevant for the question here
not_before = models.DateTimeField(auto_now_add=True)
...
class ExampleItem(models.Model):
myParent = models.ForeignKey(Example, on_delete=models.CASCADE)
execution_date = models.DateTimeField(auto_now_add=True)
....
Can I have code running/triggered whenever an ExampleItem is "added to the list of items in an Example instance"? What I would like to do is run some checks and, depending on the concrete Example instance possibly alter the ExampleItem before saving it.
To illustrate
Let's say the Example's class not_before date dictates that the ExampleItem's execution_date must not be before not_before I would like to check if the "to be saved" ExampleItem's execution_date violates this condition. If so, I would want to either change the execution_date to make it "valid" or throw an exception (whichever is easier). The same is true for a duplicate execution_date (i.e. if the respective Example already has an ExampleItem with the same execution_date).
So, in a view, I have code like the following:
def doit(request, example_id):
# get the relevant `Example` object
example = get_object_or_404(Example, pk=example_id)
# create a new `ExampleItem`
itm = ExampleItem()
# set the item's parent
itm.myParent = example # <- this should trigger my validation code!
itm.save() # <- (or this???)
The thing is, this view is not the only way to create new ExampleItems; I also have an API for example that can do the same (let alone that a user could potentially "add ExampleItems manually via REPL). Preferably the validation code must not be duplicated in all the places where new ExampleItems can be created.
I was looking into Signals (Django docu), specifically pre_save and post_save (of ExampleItem) but I think pre_save is too early while post_save is too late... Also m2m_changed looks interesting, but I do not have a many-to-many relationship.
What would be the best/correct way to handle these requirements? They seem to be rather common, I imagine. Do I have to restructure my model?
The obvious solution here is to put this code in the ExampleItem.save() method - just beware that Model.save() is not invoked by some queryset bulk operations.
Using signals handlers on your own app's models is actually an antipattern - the goal of signal is to allow for your app to hook into other app's lifecycle without having to change those other apps code.
Also (unrelated but), you can populate your newly created models instances directly via their initializers ie:
itm = ExampleItem(myParent=example)
itm.save()
and you can even save them directly:
# creates a new instance, populate it AND save it
itm = ExampleItem.objects.create(myParent=example)
This will still invoke your model's save method so it's safe for your use case.
Related
I have an Events table which contains various types of events. I care only about one of those types. As a result, every query I write begins with
Events.objects.filter(event_type="the_type").\
etc(...).etc(...)`.
Obviously this is repetitive and easy to forget. Is there a way to use a custom Manager so that the objects attribute always returns a specific subset of the rows, without explicitly asking for it? Or any other way to restrict the model to specific subset of the rows??
Yes, we can make a manager like:
from django.db import models
class EventManager(models.Manager):
def get_queryset(self):
return super(EventManager, self).get_queryset().filter(event_type="the_type")
and then add the manager to the Event class:
class Event(models.Model):
# ...
objects = EventManager()
Note however that some parts of Django will not use .objects, but ._base_manager, and thus still return the entire set. Furthermore my own experience with overriding the .objects manager is that it turns out to cause a lot of harm, for example if you want to set an attribute of all events, then writing Event.objects.all().update(foo='bar') will only update the events with the_type as type, whereas the code suggests otherwise.
Personally I think it is better to construct a manager with a different name, that at least hints that something is filtered, for example:
class Event(models.Model):
# ...
all_events = models.Manager()
type_events = EventManager()
here Event.objects no longer exist, but you write Event.all_events, or Event.type_events, and thus the code clearly hints what subset you take.
I am trying to rewrite get_by_natural_key method on django manager (models.Manager). After adding model (NexchangeModel) I can delete all() objects but single - cannot.
Can:
SmsToken.objects.all().delete()
Cannot:
SmsTokent.objects.last().delete()
Code:
from django.db import models
from core.common.models import SoftDeletableModel, TimeStampedModel, UniqueFieldMixin
class NexchangeManager(models.Manager):
def get_by_natural_key(self, param):
qs = self.get_queryset()
lookup = {qs.model.NATURAL_KEY: param}
return self.get(**lookup)
class NexchangeModel(models.Model):
class Meta:
abstract = True
objects = NexchangeManager()
class SmsToken(NexchangeModel, SoftDeletableModel, UniqueFieldMixin):
sms_token = models.CharField(
max_length=4, blank=True)
user = models.ForeignKey(User, related_name='sms_token')
send_count = models.IntegerField(default=0)
While you are calling:
SmsToken.objects.all().delete() you are calling the queryset's delete method.
But on SmsTokent.objects.last().delete() you are calling the instance's delete method.
After django 1.9 queryset delete method returns no of items deleted. REF
Changed in Django 1.9:
The return value describing the number of objects deleted was added.
But on instance delete method Django already knows only one row will be deleted.
Also note that querset's delete method and instance's delete method are different.
The delete()[on a querset] method does a bulk delete and does not call any delete() methods on your models[instance method]. It does, however, emit the pre_delete and post_delete signals for all deleted objects (including cascaded deletions).
So you cannot rely on the response of the method to check if the delete worked fine or not. But in terms of python's philosophy "Ask for forgiveness than for permission". That means you can rely on exceptions to see if a delete has worked properly the way it should. Django's ORM will raise proper exceptions and do proper rollbacks in case of any failure.
So you can do this:
try:
instance.delete()/querset.delete()
except Exception as e:
# some code to notify failure / raise to propagate it
# you can avoid this try..except if you want to propagate exceptions as well.
Note: I am catching generic Exception because the only code in my try block is delete. If you wish to have some other code then you must catch specific exceptions only.
I assume that SoftDeletableModel comes from the django-model-utils package? If so, the purpose of that model is to mark instances with an is_removed field rather than actually deleting them. So it's to be expected that calling delete() on a model instance—which is what you get from last()—wouldn't actually delete anything.
SoftDeletableModel provides an objects attribute with a manager that limits its results to non-removed objects, and overrides delete() to mark objects as removed instead of actually deleting them.
The problem is that you've defined your own manager as objects, so the SoftDeletableModel manager isn't being used. Your custom manager is actually bulk deleting objects from the database, contrary to the goal of doing a soft delete. The way to resolve this is to have your custom manager inherit from SoftDeletableManagerMixin:
class NexchangeManager(SoftDeletableManagerMixin, models.Manager):
# your custom code
Here is an abstract base class for many of my "Treatment" models (TreatmentA, TreatmentB, etc):
class TreatmentBase(models.Model):
URL_PREFIX = '' # child classes define this string
code = shared.models.common.RandomCharField(length=6)
class Meta:
abstract = True
Each Treatment instance has a URL, that when visited by a user, takes them to a page specific to that treatment. I want to be able to create a Treatment in Django Admin, and immediately get this URL so I can send it to users. This URL can be created with the following method on TreatmentBase:
def get_url(self):
return '{}/{}/'.format(self.URL_PREFIX, self.code)
However, I am stuck with how to get this URL to display in Django Admin. I can think of the following solutions:
(1) Customize the display of the code field so that it becomes a clickable URL. Problem: I don't know how to do this.
(2) Add the get_url method to ModelAdmin.list_display. Problem: This means I would have to define a separate list_display for each of the child models of BaseTreatment, and I would have to explicitly list all the fields of the model, meaning I have to update it every time I modify a model, violating DRY.
(3) Add an extra field like this:
url = models.URLField(default = get_url)
Problem: get_url is an instance method (since it needs to refer to the self.code field), and from my reading of the docs about the default argument, it just has to be a simple callable without arguments.
Any way to do this seemingly simple task?
You could go with option 2 (adding to the admin display) but add it to the
readonly_fields which may alleviate your DRY concerns when models changes.
Option 3 (the extra field) could also work if you override the save method setting the URL property. You'd either want to set the field as readonly in the admin or only set the value in the save method if it's currently None.
I've been working through an ordered ManyToManyField widget, and have the front-end aspect of it working nicely:
Unfortunately, I'm having a great deal of trouble getting the backend working. The obvious way to hook up the backend is to use a through table keyed off a model with ForeignKeys to both sides of the relationship and overwrite the save method. This would work great, except that due to idiosyncrasies of the content, it is an absolute requirement that this widget be placed in a fieldset (using the ModelAdmin fieldsets property), which is apparently not possible.
I'm out of ideas. Any suggestions?
Thanks!
In regard to how to set up the models, you're right in that a through table with an "order" column is the ideal way to represent it. You're also right in that Django will not let you refer to that relationship in a fieldset. The trick to cracking this problem is to remember that the field names you specify in the "fieldsets" or "fields" of a ModelAdmin do not actually refer to the fields of the Model, but to the fields of the ModelForm, which we are free to override to our heart's delight. With many2many fields, this gets tricky, but bear with me:
Let's say you're trying to represent contests and competitors that compete in them, with an ordered many2many between contests and competitors where the order represents the competitors' ranking in that contest. Your models.py would then look like this:
from django.db import models
class Contest(models.Model):
name = models.CharField(max_length=50)
# More fields here, if you like.
contestants = models.ManyToManyField('Contestant', through='ContestResults')
class Contestant(models.Model):
name = models.CharField(max_length=50)
class ContestResults(models.Model):
contest = models.ForeignKey(Contest)
contestant = models.ForeignKey(Contestant)
rank = models.IntegerField()
Hopefully, this is similar to what you're dealing with. Now, for the admin. I've written an example admin.py with plenty of comments to explain what's happening, but here's a summary to help you along:
Since I don't have the code to the ordered m2m widget you've written, I've used a placeholder dummy widget that simply inherits from TextInput. The input holds a comma-separated list (without spaces) of contestant IDs, and the order of their appearance in the string determines the value of their "rank" column in the ContestResults model.
What happens is that we override the default ModelForm for Contest with our own, and then define a "results" field inside it (we can't call the field "contestants", since there would be a name conflict with the m2m field in the model). We then override __init__(), which is called when the form is displayed in the admin, so we can fetch any ContestResults that may have already been defined for the Contest, and use them to populate the widget. We also override save(), so that we can in turn get the data from the widget and create the needed ContestResults.
Note that for the sake of simplicity this example omits things like validation of the data from the widget, so things will break if you try to type in anything unexpected in the text input. Also, the code for creating the ContestResults is quite simplistic, and could be greatly improved upon.
I should also add that I've actually ran this code and verified that it works.
from django import forms
from django.contrib import admin
from models import Contest, Contestant, ContestResults
# Generates a function that sequentially calls the two functions that were
# passed to it
def func_concat(old_func, new_func):
def function():
old_func()
new_func()
return function
# A dummy widget to be replaced with your own.
class OrderedManyToManyWidget(forms.widgets.TextInput):
pass
# A simple CharField that shows a comma-separated list of contestant IDs.
class ResultsField(forms.CharField):
widget = OrderedManyToManyWidget()
class ContestAdminForm(forms.models.ModelForm):
# Any fields declared here can be referred to in the "fieldsets" or
# "fields" of the ModelAdmin. It is crucial that our custom field does not
# use the same name as the m2m field field in the model ("contestants" in
# our example).
results = ResultsField()
# Be sure to specify your model here.
class Meta:
model = Contest
# Override init so we can populate the form field with the existing data.
def __init__(self, *args, **kwargs):
instance = kwargs.get('instance', None)
# See if we are editing an existing Contest. If not, there is nothing
# to be done.
if instance and instance.pk:
# Get a list of all the IDs of the contestants already specified
# for this contest.
contestants = ContestResults.objects.filter(contest=instance).order_by('rank').values_list('contestant_id', flat=True)
# Make them into a comma-separated string, and put them in our
# custom field.
self.base_fields['results'].initial = ','.join(map(str, contestants))
# Depending on how you've written your widget, you can pass things
# like a list of available contestants to it here, if necessary.
super(ContestAdminForm, self).__init__(*args, **kwargs)
def save(self, *args, **kwargs):
# This "commit" business complicates things somewhat. When true, it
# means that the model instance will actually be saved and all is
# good. When false, save() returns an unsaved instance of the model.
# When save() calls are made by the Django admin, commit is pretty
# much invariably false, though I'm not sure why. This is a problem
# because when creating a new Contest instance, it needs to have been
# saved in the DB and have a PK, before we can create ContestResults.
# Fortunately, all models have a built-in method called save_m2m()
# which will always be executed after save(), and we can append our
# ContestResults-creating code to the existing same_m2m() method.
commit = kwargs.get('commit', True)
# Save the Contest and get an instance of the saved model
instance = super(ContestAdminForm, self).save(*args, **kwargs)
# This is known as a lexical closure, which means that if we store
# this function and execute it later on, it will execute in the same
# context (i.e. it will have access to the current instance and self).
def save_m2m():
# This is really naive code and should be improved upon,
# especially in terms of validation, but the basic gist is to make
# the needed ContestResults. For now, we'll just delete any
# existing ContestResults for this Contest and create them anew.
ContestResults.objects.filter(contest=instance).delete()
# Make a list of (rank, contestant ID) tuples from the comma-
# -separated list of contestant IDs we get from the results field.
formdata = enumerate(map(int, self.cleaned_data['results'].split(',')), 1)
for rank, contestant in formdata:
ContestResults.objects.create(contest=instance, contestant_id=contestant, rank=rank)
if commit:
# If we're committing (fat chance), simply run the closure.
save_m2m()
else:
# Using a function concatenator, ensure our save_m2m closure is
# called after the existing save_m2m function (which will be
# called later on if commit is False).
self.save_m2m = func_concat(self.save_m2m, save_m2m)
# Return the instance like a good save() method.
return instance
class ContestAdmin(admin.ModelAdmin):
# The precious fieldsets.
fieldsets = (
('Basic Info', {
'fields': ('name', 'results',)
}),)
# Here's where we override our form
form = ContestAdminForm
admin.site.register(Contest, ContestAdmin)
In case you're wondering, I had ran into this problem myself on a project I've been working on, so most of this code comes from that project. I hope you find it useful.
I have a situation where I need to notify some users when something in DB changes. My idea is to catch pre_save and post_save signal and make some kind of diff and mail that. Generally it works good, but I don't know how to get diff for m2m fields.
At the moment I have something like this:
def pre_save(sender, **kwargs):
pk = kwargs['instance'].pk
instance = copy.deepcopy(sender.objects.get(pk=pk))
tracking[sender] = instance
def post_save(sender, **kwargs):
instance = copy.deepcopy(kwargs['instance'])
print diff(instance, (tracking[sender])) # TODO: don't print, save diff somewhere
Diff function should work for every model (at the mommet I have four model classes). With deepcopy I can save old model, but I don't know how to save m2m fields because they are in separate table (yes, I know I can get this data, but at the momment of execution I don't know what fields are m2m and I wouldn't like to create different slot for every model). What I would like is generic solution, so I can just add models later without thinking about notification part.
My plan is to call get_data() and clear_data() functions after save() in view to clean diff that slots have generated.
Is this good way of doing this? Is there a better way? Is there django application that can do this job for me?
Excuse my English, it's not my native language.
First of all, you don't need to use deepcopy for this. Re-querying the sender from the database returns a "fresh" object.
def pre_save(sender, **kwargs):
pk = kwargs['instance'].pk
instance = sender.objects.get(pk=pk)
tracking[sender] = instance
You can get a list of all the many-to-many fields for a class, and check the values related to the current instance:
for field in sender._meta.local_many:
values = field.value_from_object(instance).objects.all()
# Now values is a list of related objects, which you can diff