How to add sub field count to django ORM model? [duplicate] - django

Question

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How to perform a join and aggregate count in Django
(1 answer)
Closed 4 years ago.
I am trying to send sub model count information with the main model to HTML template. I have PROJECT and Companies models:
class Projects(models.Model):
name = models.CharField(max_length=255)
note = models.CharField(max_length=255, default='')
def __str__(self):
return self.name
class Companies(models.Model):
project = models.ForeignKey(Projects, on_delete=models.CASCADE)
name = models.CharField(max_length=255)
note = models.CharField(max_length=255, default='')
def __str__(self):
return self.name
I need to show, projects have how many companies(company count):
Project name | company.no
project1 | 3
project2 | 5

Answer 1

Method-1 : Using a GROUP BY statement using Django ORM
from django.db.models import Count
Projects.objects.values('name').annotate(count=Count('companies'))
This is equivalant to the SQL query, SELECT name,Count('companies') as count from Projects. This will return a QuerySet as,
<QuerySet [{'name': 'project_name_1', 'count': 10}, {'name': 'project_name_2', 'count': 6}]>
Method-2 : Use #property decorator (as #Kirollos Morkos mentioned )
Hence your model became
class Projects(models.Model):
name = models.CharField(max_length=255)
note = models.CharField(max_length=255, default='')
#property
def company_count(self):
return self.companies_set.count()
def __str__(self):
return self.name
Then you'll get the comapny count in template by {{ project_obj.company_count}}
NOTE: The Method-1 is more efficent and fast, because it's being done in database level

Answer 2

You can use the property decorator to provider an arbitrary getter on your Project model. In your Company model, add a related_name to the project foreign key so you can get the reverse relationship:
project = models.ForeignKey(Projects, on_delete=models.CASCADE, related_name='companies')
Then, you can add a property on your Project model to get the company count:
class Projects(models.Model):
#property
def company_count(self):
return self.companies.count()
...
# Following the sample counts you gave
print(project1.company_count) # 3
print(project2.company_count) # 5

Answer 3

As JPG says you can use annotate. you dont need to add values queryset. see below:
from django.db.models import Count
Projects.objects.filter(<any filter apply to queryset>).annotate(company_count=Count('companies'))
you can add any field to annotation. then in your template or any code you can use this count by calling:
project.company_count
you also can add in your template
{{ project.companies_set.count }}
to get count of companies. but this method is too slow. because for each project record you have a database hit.