How to count items in a foreign key?

问题

I am making a blog app using django...

This is my model.py

class categories(models.Model):
    Title = models.CharField(max_length=40)

class Blog(models.Model):
    User = models.ForeignKey(settings.AUTH_USER_MODEL,on_delete=models.CASCADE,null=True,blank=True)
    Date = models.DateTimeField(default=datetime.now)
    Blog_title = models.CharField(max_length=255)
    likes = models.ManyToManyField(settings.AUTH_USER_MODEL,related_name='likes',blank=True)
    Description = RichTextUploadingField(blank=True, null=True,config_name='special')
    Blog_image = models.ImageField(upload_to='blog_image', null=True, blank=True)
    Category = models.ForeignKey(categories,on_delete=models.CASCADE,related_name='Categories')

I was wondering How to count the total no of blog present under a particular category?

I want to track a specific count rate for all Categories...

回答1:

If you want to count the number of Blogs per Category, we can make an annotation on the Category:

from django.db.models import Count

Category.objects.annotate(
    num_blogs=Count('Categories')
)

But (as specified later), using categories as related name is downright wrong, and should be renamed to blogs since this is the reverse relation, in that case the query is:

from django.db.models import Count

Category.objects.annotate(
    num_blogs=Count('blogs')
)

The result is a QuerySet containing all the Category objects, and each Category object, will have an attribute num_blogs that contains the number of blogs related to this category.

The advantage of doing this by annotations is that you avoid the "N+1 problem": here we still perform one query, and not one query for the categories, and n queries to obtain the blog count for that specific category. In MySQL the query will look like:

SELECT category.*, COUNT(blog.id)
FROM category
LEFT OUTER JOIN blog ON blog.category_id = category.id
GROUP BY category.id

Note: typically attributes in Python start wit a lowercase (and use underscores between words), so blog_title, and category instead of Blog_title and Category

Note: the related_name is the name you give the objects of the reverse relation, so the related_name of the category = ForeignKey(..) should be 'blogs', not 'categories'.

回答2:

for title in categories.objects.all():
    Blog.objects.filter(Category__Title=a.title).count()

This would do what you are looking for.

回答3:

[{"category_title":category.Title, "count":Blog.objects.filter(Category=category).count()} for category in categories.objects.all()]

This will give a list of dictionaries with the category title and its corresponding counts

回答4:

you can do with Category model also.

Categories.objects.filter(Title="someName").Categories.all().count()

来源：https://stackoverflow.com/questions/52418944/how-to-count-items-in-a-foreign-key

标签

django

django-models

foreign-keys