RelatedManager not returning excepted results - django

Question

I am currently working on a django app where I have two models:
class Job(models.Model):
def __unicode__(self):
return self.job_number
area = models.ForeignKey(Area)
customer = models.ForeignKey(Customer)
job_address = models.ForeignKey(Address, null=True)
job_status = models.ForeignKey(JobStatus)
surveyor = models.ForeignKey(Employee, null=True)
job_dates = models.OneToOneField(JobDates, null=True)
job_title = models.CharField(max_length=50, null=False)
job_number = models.CharField(max_length=15, null=False, unique=True)
deposit_amount = models.DecimalField(null=True, decimal_places=2, max_digits=10)
is_deposit_paid = models.BooleanField(default=False)
is_deposit_required = models.BooleanField(default=False)
particular = models.SmallIntegerField(null=True)
description = models.TextField(null=True)
surveyor_note = models.TextField(null=True)
contractor_note = models.TextField(null=True)
class Picture(models.Model):
def __unicode__(self):
return self.file_name
job = models.ForeignKey(Job, related_name='picture_job_set')
url = models.CharField(max_length=450, null=False, unique=True)
file_name = models.CharField(max_length=50, null=False)
is_main_pic = models.BooleanField(default=False)
title = models.CharField(max_length=50, null=True)
description = models.TextField(null=True)
date_uploaded = models.DateTimeField(null=False)
uploaded_by = models.ForeignKey(Employee, null=False)
So it is a one to many relationship where each job can have many pictures associated with it. Now in the picture model, I have the "is_main_pic" column which is used to display a picture in a listview and when they click on it, it goes to details and shows the rest of the pictures. Now my problem is whenever I try to filter to just get the main picture within each job in the list view, it returns all the pictures for each job in the list view if the condition is met for any of the pictures. My query looks like this:
Jobs.objects.filter(picture_job_set__is_main_pic=True).prefetch_related('picture_job_set')
Now when I go to access the picture_job_set, it contains all pictures associated with a job, even if their "is_main_pic" column is false.
I would like to be able to in the template view just to call first on the picture_job_set as there will always only be one main picture for each job. As of right now I am able to get around this by simply having a for loop in the template with an if statement to determine which is the main pic but this seems messy and inefficient.
Am I misunderstanding how RelatedManager works? Do I have to call two queries to achieve this? Coming from a ASP MVC background, I could achieve this with something like:
List<JobIndexViewModel> viewModel = await db.Jobs
.Select(x => new JobIndexViewModel
{
JobID = x.JobID,
JobTitle = x.JobTitle,
JobNumber = x.JobNumber,
BidExpireDate = x.JobDates.BidExpireDate,
JobPostedDate = x.JobDates.JobPostedDate,
City = x.Address.City,
PictureUrl = x.Pictures.Where(t => t.IsMainPic == true).Select(t => t.Url).FirstOrDefault()
}).ToListAsync();

Answer 1

I, playing django now, interprete your query as:
Give me all Jobs that has a picture from picture_job_sets, inasmuch as
one of the pictures is marked as main picture.
So like you said, it returns all the pictures for each job in the list view if the condition is met for any of the pictures.
Quoting you: there will always only be one main picture for each job. If there will always be a main picture, you can easily start the query from the pictures:
Picture.objects.filter(is_main_pic=True).select_related('Job')
Then you can iterate over each picture and do .job on each to get the related job.
One last thing though, I suspect you have this problem because of your design. Won't you have a simpler design if you move a main picture as a field to your Job model?