Django models - create items which have to pass various tasks - django

Question

I am trying to create a model for the following scenario:
An item is delivered and has to pass through various tasks in a certain order.
As soon as the item has passed through a task, the task is marked as completed.
The names of the tasks are known in advance.
For example:
Create item
task 1 'register the item'
task 2 'install the item'
task 3 'check logs of item'
I started with a simple model like this, but it looks very static to me and not really i what i am looking for.
models.py
class Item(models.Model):
item_id = models.PositiveIntegerField()
item_priority = models.PositiveSmallIntegerField(default=2)
item_date_created = models.DateField(auto_now_add=True)
task1_name = models.CharField(max_length=128)
task1_done_by = models.CharField(max_length=64, blank=True)
task1_done_date = models.DateField(null=True, blank=True)
task2_name = models.CharField(max_length=128)
task2_done_by = models.CharField(max_length=64, blank=True)
task2_done_date = models.DateField(null=True, blank=True)
# ... next 20 tasks
def __str__(self):
return str(self.item_id)
Trying Relational fields, for example a ManyToMany Field to pre define the tasks, i ended up with this:
class Task(models.Model):
name = models.CharField(max_length=128)
done_by = models.CharField(max_length=64, blank=True)
done_date = models.DateField(null=True, blank=True)
class Item(models.Model):
item_id = models.PositiveIntegerField()
item_priority = models.PositiveSmallIntegerField(default=2)
item_date_created = models.DateField(auto_now_add=True)
tasks = models.ManyToManyField(Task)
But, if i create two items with the same tasks and mark the task of the first item as done it will be marked as done in the second item as well -- this is not what i want.
How to assign a task to a specific item? Or which other model would best fit my scenario?

Answer 1

For executing code at object creation time you generally override the save() method. So when an Item gets created, in the save() method you create and link dynamically the Tasks to Item.tasks
class Item(models.Model):
item_id = models.PositiveIntegerField()
item_priority = models.PositiveSmallIntegerField(default=2)
item_date_created = models.DateField(auto_now_add=True)
# remove m2m !!!
def save(self, *args, **kwargs):
# If it has no pk then it is a *new* item
if not self.pk:
# Save by manually
# Call save of parent class (models.Model) creating the custom class
super(Item, self).save(*args, **kwargs)
# Now you have the PK
t1 = Task( name='T1', item=self )
t1.save()
#...
class Task(models.Model):
name = models.CharField(max_length=128)
done_by = models.CharField(max_length=64, blank=True)
done_date = models.DateField(null=True, blank=True)
# Add Item FK
item = models.ForeignKey(Item)
Note on Code Update:
1) I changed the direction: the task links to the item, not the other way round. See https://docs.djangoproject.com/en/1.11/topics/db/examples/many_to_one/
2) I still check if it is a new instance, then call super save and then I create the task for this item.
Make sure to check the self.pk value otherwise the code will get executed also when updating the object.
Also be aware of that you have to create a new Task object linked to an Item object every time. If you link one task object to two items then when item_a completes a task the task would appear to be completed also for item_b.
What you need to do is create task1 for item_a and task2 for item_b. This way you separate the tasks.
Read on:
https://docs.djangoproject.com/en/1.11/ref/models/instances/#saving-objects
https://docs.djangoproject.com/en/1.11/topics/db/models/#relationships