Django computed column with lookup from same table and if class - django

Question

I am new to programming. I am trying to create a project management site. I created a model as follows:
class Boqmodel(models.Model):
code = models.IntegerField(unique=True)
building = models.ForeignKey(building, on_delete=models.SET_NULL, null=True)
level = models.ForeignKey(level, on_delete=models.SET_NULL, null=True)
activity = models.ForeignKey(activity, on_delete=models.SET_NULL, null=True)
subactivity = models.ForeignKey(sub_activity, on_delete=models.SET_NULL, null=True)
duration = models.IntegerField()
linkactivity = models.IntegerFeild(null=True) #contains code (same as code field) which this specific code is linked to
linktype = models.CharField(max_length=300, null=True)# only two choices start or finish
linkduration = models.IntegerField(null=True)
plannedstart = models.DateField()
plannedfinish = models.DateField()
The problem is I need my planned start as a computed column. The planned column should be as follows:
if linkactivity is null, then it should take a default value 01-01-2019
if else then it should look the linkactivity in code field and then
if linktype is start, then it should specify the start date of code activity +duration
or if linktype is finish plannedfinish + duration
Example
First entry:
code=1
building=A-1
Level=L-1
Activity=Activity-1
Subactivity-Subactivity-1
duration=2
linkactivity=null
linktype=null
linkduration=null
planned start=01-01-2019(as linkactivity=null)
plannedfinish=03-01-2019(planned start+duration)
Second entry:
code=2
building=A-1
Level=L-1
Activity=Activity-2
Subactivity-Subactivity-2
duration=3
linkactivity=1
linktype=start
linkduration=1
planned start=02-01-2019(as linkactivity=1,it searches code1 ,as linktype=start,it searches startdate of code 1 it is 01-01-2019 ; finally 01-01-2019+link duration(1)=02-01-2019)
plannedfinish=05-01-2019(planned start+duration)
Any help would be really appreciated

Answer 1

When you call Model.objects.create() method, the django ORM implicitly calls the Model.save() method inside your model. Since, you are inheriting BoqModel from models.Model and there is no explicit BoqModel.save() method in your BoqModel, Model.objects.create() method calls the save() method from the parent class. Inside Model.save() method you can customize your model save behavior. In order to achieve your intended action, override the save() method as follows -
class Boqmodel(models.Model):
code = models.IntegerField()
building = models.ForeignKey(building, on_delete=models.SET_NULL, null=True)
level = models.ForeignKey(level, on_delete=models.SET_NULL, null=True)
activity = models.ForeignKey(activity, on_delete=models.SET_NULL, null=True)
subactivity = models.ForeignKey(sub_activity, on_delete=models.SET_NULL, null=True)
duration = models.IntegerField()
linkactivity = models.IntegerFeild(null=True) #contains code (same as code field) which this specific code is linked to
linktype = models.CharField(max_length=300, null=True)# only two choices start or finish
linkduration = models.IntegerField(null=True)
plannedstart = models.DateField()
plannedfinish = models.DateField()
def save(self, *args, **kwargs):
if self.linkactivity is None:
self.plannedstart = datetime.date(year=2019, month=1, day=1)
else:
boq = BoqModel.objects.get(code=self.linkactivity)
if self.linktype is not None:
if self.linktype == "start":
self.plannedstart = boq.plannedstart + datetime.timedelta(days=self.linkduration)
elif self.linktype == "finish":
self.plannedstart = boq.plannedfinish + datetime.timdelta(days=self.linkduration)
else:
self.plannedstart = datetime.date(year=2019, month=1, day=1)
else:
self.plannedstart = datetime.date(year=2019, month=1, day=1)
self.plannedfinish = self.plannedstart + datetime.tiemdelta(days=self.linkduration)
super(BoqModel, self).save(*args, **kwargs)
The above is not optimized. But you get the general idea of how to do it. Also learn more about overriding predefined model methdos here.