Django 查询集在查询中获取空值

Question

So I'm trying to get all the values to be able to display in my templates. However I have encountered myself with this problem:

I have these models:

class Project(models.Model):
    projectId = models.AutoField(primary_key=True, db_column="Id", db_index=True, verbose_name='Project ID')
    description = models.CharField(max_length=900)

class PM(models.Model):
    PMid = models.AutoField(primary_key=True, db_column="Id", db_index=True)
    PMNumber = models.CharField(max_length=50, unique=True, db_column="PMNumber")
    description = models.CharField(max_length=600)
    projectId = models.ForeignKey(Project, on_delete=models.CASCADE,
                                  db_column="ProjectID", related_name='+')

And I'm trying to get all the projects with their respective PMNumber. For which I'm trying something like this:

fpm = PM.objects.all()
projects = []
for i in fpm:
   projects.append({'ProjectName': i.projectId.description, 'PMNumber': i.PMNumber})

After that the result is like this

[{'ProjectName': 'pr1', 'PMNumber': '119508-01'}, {'ProjectName': 'pr1', 'PMNumber': '119490-01'}]

However, I'm still missing some projects that don't have a PM related to them, I want all the projects in the queryset to be able to show them in a template, not just the ones that have a PM. It should look something like this:

[{'ProjectName': 'pr2', 'PMNumber':'None'}, {'ProjectName':'pr3' , 'PMNumber':'None'}, {'ProjectName': 'pr1', 'PMNumber': '119508-01'}, {'ProjectName': 'pr1', 'PMNumber': '119490-01'}]

Is there a way to do this? or is there another way to do this in the views?

Note: I know that setting an ID for each class in Django is not right, but it's kind of a standard in the place where I work at. So I can't change it.

Answer 1

You should then do this the opposite way:

result = list(Project.objects.values(
    'description',
    
))

For the Project s for which no PM exists, None will be used for ProjectName . If there are multiple PM s for the same Project , all these PM s will be added.

That being said, I do not see why you set related_name to realted_name='+' . This is the name of the relation in reverse, so likely it is better to set it to something like:

class PM(models.Model):
    PMid = models.AutoField(primary_key=True, db_column="Id", db_index=True)
    PMNumber = models.CharField(max_length=50, unique=True, db_column="PMNumber")
    description = models.CharField(max_length=600)
    projectId = models.ForeignKey(
        Project, on_delete=models.CASCADE,
        db_column="ProjectID",
        
)

Then you query with:

result = list(Project.objects.values(
    'description',
    
))

You can replace a missing ProjectName with the description of the project with:

from django.db.models.functions import Coalesce

result = list(Project.objects.values(
    'description',
    
))

You can also post-process the list to a dictionary that contains dictionaries where the projectName (or perhaps better projectNames is a list , with:

from itertools import groupby
from operator import itemgetter
from django.db.models.functions import Coalesce

result = list(Project.objects.values(
    'description',
    
))

result = [ 
    { 'description': k, 'projectNames': list(map(itemgetter('projectName'), vs)) }
    for k, vs in groupby(itemgetter('description'), result)
]

or we can make an empty list for Projects with no PM with:

from itertools import groupby
from operator import itemgetter

result = list(Project.objects.values(
    'description',
    
))

result = [ 
    { 'description': k, 'projectNames': list(filter(None, map(itemgetter('projectName'), vs))) }
    for k, vs in groupby(itemgetter('description'), result)
]