How to rearrange priority field for a django model? - django

Question

I have a Model with a priority field of type postitive integer. This field is unique and allows me to manage the priority of objects.
For example, I want the most important object to have priority one, the second most important to have priority two, etc...
Example:
[
{ "name": "object82",
"priority": 1
}
{ "name": "object54",
"priority": 2
}
{ "name": "object12",
"priority": 3
}
]
class MyObject(models.Model):
name = models.CharField(_("name"), max_length=255)
priority = models.PositiveSmallIntegerField(_("priority"), unique=True)
I want to override the object serializer so that if I add a new object with an existing priority, it unpacks the existing objects. (same thing for the path of an existing object)
For example if I take the example above and add:
{ "name": "object22",
"priority": 2
}
I want the following result:
[
{ "name": "object82",
"priority": 1 // the priority didn't changed
}
{ "name": "object22", // my new object
"priority": 2
}
{ "name": "object54",
"priority": 3 // the priority had changed
}
{ "name": "object12", // the priority had changed
"priority": 4
}
]
I think I have to check first if an object with the same priority exists in the database or not.
If not => I save as is
If yes, I have to change the priority of some objects before add the new object.
How to do this ?
Maybe something like:
class MyObjectSerializer(serializers.ModelSerializer):
class Meta:
model = MyObject
fields = '__all__'
def update(self, instance, validated_data):
target_priority = validated_data.get('priority')
if MyObject.objects.filter(target_priority).exists():
existing_priorities = MyObject.objects.filter(priority__gte=target_priority)
for existing_priority in existing_priorities:
existing_priority.priority += 1
existing_priority.save(update_fields=['priority'])
instance.priority = target_priority
instance.save(update_fields=['priority'])

Answer 1

I was facing a similar problem, and what I have done is that I have a model form and I'm doing the validation in clean function
def clean(self):
cleaned_data = super().clean()
priority = cleaned_data.get('priority')
task = Task.objects.filter(priority__exact=priority)
while task.exists():
prev_task_id = task[0].id
task.update(priority=priority+1)
priority += 1
task = Task.objects.filter(priority__exact=priority).exclude(pk=prev_task_id)
return cleaned_data
I have used the prev_task_id variable for excluding the model that is just got updated. For e.g. let's say we have data
{
title: 'first one',
priority: 3
},
{
title: 'second one',
priority: 4
}
So now if I got priority 3 and after updating it we will have two tasks with priority 4 so we have to exclude the previous task i.e. 'first one'. We have to only update the second task in next iteration
PS:- This code is written assuming that in the database no duplicate priority exists.