Update datetimefiled of all related models when model is updated - django

I have two models (Post and Display). Both have Datetime-auto fields. My problem is that i want to update all display objects related to a post, once a post is updated.
I have read here that you could override one models save method, but all the examples are About updating the model with the foreign key in it and then call the save method of the other model. In my case it's the other way arround. How can i do this ?
class Post(models.Model):
title = models.CharField(max_length=40)
content = models.TextField(max_length=300)
date_posted = models.DateTimeField(auto_now=True)
author = models.ForeignKey(User, on_delete=models.CASCADE)
rooms = models.ManyToManyField(Room, related_name='roomposts', through='Display')
def __str__(self):
return self.title
def get_absolute_url(self):
return "/post/{}/".format(self.pk)
class Display(models.Model):
post = models.ForeignKey(Post, on_delete=models.CASCADE)
room = models.ForeignKey(Room, on_delete=models.CASCADE)
isdisplayed = models.BooleanField(default=0)
date_posted = models.DateTimeField(auto_now=True)
def __str__(self):
return str(self.isdisplayed)
i want to update the date_posted of all related Display-objects once their related post is changed. I do not know if overriding the save-method works here.

in this case you should have a look at django's reverse foreign key documentation
https://docs.djangoproject.com/en/2.2/topics/db/queries/#following-relationships-backward
in your case you can override the save method on your Post model
def save(self, *args, **kwargs):
super().save(*args, **kwargs)
#either: this excutes many sql statments
for display in self.display_set.all():
display.save()
#or faster: this excute only one sql statements,
#but note that this does not call Display.save
self.display_set.all().update(date_posted=self.date_posted)
The name display_set can be changed using the related_name option
in Display, you can change it:
post = models.ForeignKey(Post, on_delete=models.CASCADE, related_name='displays')
Then, instead of using self.display_set in your code, you can use self.displays

Overriding save method works, but that's not were you should go, imo.
What you need is signals:
#receiver(post_save, sender=Post)
def update_displays_on_post_save(sender, instance, **kwargs):
if kwargs.get('created') is False: # This means you have updated the post
# do smth with instance.display_set
Usually it goes into signals.py.
Also you need to include this in you AppConfig
def ready(self):
from . import signals # noqa

Related

Django two table relationship, but not both

I have a table called Post. A post can have 2 videos or 2 images, but not both. The table schema for a post looks like this:
class Post(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE)
header = models.CharField()
created_at = models.DateTimeField(auto_now_add=True)
I have two tables that look similar to each other:
class PostImage(models.Model):
post = models.ForeignKey(Post, on_delete=models.CASCADE)
img = models.ImageField()
class PostVideo(models.Model):
post = models.ForeignKey(Post, on_delete=models.CASCADE)
video = models.FileField()
How do I create and enforce the relationship where a post can have maximum and minimum of 2 images or 2 videos, but it can't have both videos and images at the same time? Or is there a better way to do this?
Probably you can write a Mixin class like this:
class PostMixin(object):
def save(self, *args, **kwargs):
if hasattr(self, 'img') and self.post.images.exists():
raise ValidationError('Already have an image')
elif hasattr(self, 'video') and self.post.videos.exists():
raise ValidationError('Already have a video')
super(PostMixin, self).save(*args, **kwargs)
class PostImage(PostMixin, models.Model):
post = models.ForeignKey(Post, on_delete=models.CASCADE, related_name="images")
img = models.ImageField()
class PostVideo(PostMixin, models.Model):
post = models.ForeignKey(Post, on_delete=models.CASCADE, related_name="videos")
video = models.FileField()
But rather than handling them in model, you should handle them in forms or serializers(if you are using DRF).
Another method is to add a pre_save signal for both PostImage and PostVideo and check your conditions there:
#receiver(pre_save, sender=PostVideo)
#receiver(pre_save, sender=PostImage)
def post_validator(sender, instance, *args, **kwargs):
images_count = instance.post.postimage_set.count()
videos_count = instance.post.postvideo_set.count()
if not (<your conditions met>):
raise ValidationError('Conditions not met!')

Django - Update integer field value in one model by change in other

Here is a project I've created to practice, in my models.py,
class Post(models.Model):
title = models.CharField(max_length = 140)
author = models.ForeignKey(User, on_delete=models.CASCADE)
votes = models.BigIntegerField(default=0, blank=True)
class Vote(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE, related_name='voter')
post = models.ForeignKey(Post, on_delete=models.CASCADE, related_name='vpost')
#receiver(post_save, sender=Vote)
def update_votes(sender, **kwargs):
# # ??
Here I have a Voteform with that user can vote any particular post. That part works well.
Here is my question, whenever a user votes a particular post, I want votes field in Post model to increase as well.
I know I can show it with {{post.vpost.count}} in my html. But I want that increment here.
Other way I have tried,
class Vote(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE, related_name='voter')
post = models.ForeignKey(Post, on_delete=models.CASCADE, related_name='vpost')
def update_votes(self):
p = self.post
p.votes += 1
p.save()
This one only works once, not working from second time, so I want to use signal method. So how can I update the vote field in Post model using signal?
Nearly there. I would rename Post.votes to Post.votes_count as votes indicates a reverse relationship.
#receiver(post_save, sender=Vote)
def update_votes(sender, instance, **kwargs):
post = instance.post
post.votes_count += 1
post.save()
Although you might want to make sure that the count is correct, by introducing another query:
#receiver(post_save, sender=Vote)
def update_votes(sender, instance, **kwargs):
post = instance.post
post.votes_count = post.votes_set.all().count()
post.save()
You might also want to do this when/if a Vote is deleted to make sure the count is correct.
Bear in mind you could also just do this in the Vote's save method instead of needing signals.
You could also do this as a cronjob or task depending on your circumstances

Django change foreign key data and save

I have two models like
class Reporter(models.Model):
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=30)
email = models.EmailField()
class Article(models.Model):
headline = models.CharField(max_length=100)
pub_date = models.DateField()
reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)
Now for an object of Article lets say
a=Article.objects.filter(id=1)
a=a[0]
I try to change the headline and the email of the author who has written this article so I do
a.heagline = "Saving foreign key does not work"
a.reporter.email = "changed#email.com"
a.save()
This saves the Article object but does not modify the Reporter.
I explicitly have to do
a.reporter.save()
to see the changes to the reporter object. As this is a Many to One relation it should also modify the Foreign key on saving
How can I save the parent Model too by just calling the childs save method
You could override the save method or just create a custom method.
class Article(models.Model):
...
# Overriding save
def save(self, *args, **kwargs):
self.reporter.save()
super(Article, self).save(*args, **kwargs)
# Creating a custom method
def save_related(self):
self.reporter.save()
self.save()
I suggest you create a custom method because it doesn't introduce unexpected behavior in save()

Why doesn't my save method work in the admin?

In my model I overwrite the save-method for my blog model to auto-populate the slug field using the slugify method.
class BlogPost(models.Model):
title = models.CharField(max_length=100,unique=True)
slug = models.SlugField(max_length=100,unique=True)
date = models.DateTimeField(auto_now_add=True)
author = models.ForeignKey(Author)
body = models.TextField()
category = models.ForeignKey(BlogCategory)
def save(self, *args, **kwargs):
if not self.id:
# Newly created object, so set slug
self.slug = slugify(self.title)
super(BlogPost, self).save(*args, **kwargs)
But creating a new object in the admin interface doesn't work without either setting the slug field manually or doing something like
class BlogPostAdmin(admin.ModelAdmin):
prepopulated_fields = {"slug": ("title",)}
Basically I currently have the same functionality defined twice. Any ideas on how to avoid this? And: why doesn't work my own save method in the admin?
You should put blank=True in the definition of the slug field.

Adding custom field and updating model problem in django

I need to add a colorpicker to my django model and wrote a custom widget. However when I add this colordfield to my model, django gives this error:
column mediaplanner_ievent.color does not exist
LINE 1: ...nt"."bits", "mediaplanner_ievent"."capture_link", "mediaplan...
My model is :
from mediaplanner.custom_widgets import ColorPickerWidget
class ColorField(models.CharField):
def __init__(self,*args, **kwargs):
kwargs['max_length'] = 10
super(ColorField, self).__init__(*args, **kwargs)
def formfield(self, **kwargs):
kwargs['widget'] = ColorPickerWidget
return super(ColorField, self).formfield(**kwargs)
class iEvent(models.Model):
name = models.CharField(verbose_name= u"Uygulama Adı", max_length=100, unique=True)
bits = models.CommaSeparatedIntegerField(verbose_name= u"Bitler",max_length=100)
capture_link = models.URLField(verbose_name= u"Capture URL", null=True, blank=True)
color = ColorField(blank=true)
class Meta:
verbose_name = u"red button"
verbose_name_plural = u"red buttonlar"
def __unicode__(self):
return smart_str( "%s"% self.name )
The strange thing is, when I looked my database, there exist colorfield. I don't want to delete the db and load it again. But ofcourse if it's the only solution, then no choice ..
So someone can help me how to solve it?
The field in your database is named colorfield bu the field in your model is named color. You have to change one or the other to make it work again.