How to restrict models foreign keys to foreign objects having the same property
Here is my example :
We have printers. We can define page formats that are linked to a specific printer then we define workflo开发者_Python百科ws that select a starting format (first page added to the printing job), a body format and an end format (last page added to the printing job).
Start and End are not required (null and blank = True)
I want to be sure that start, body and end will be formats of the same printer.
class Printer(models.Model):
name = models.CharField(max_length = 20)
class Format(models.Model):
name = models.CharField(max_length = 20)
format = models.TextField()
printer = models.ForeignKey(Printer)
class Workflow(models.Model):
name = models.CharField(max_length = 20)
startfmt = models.ForeignKey(Format, related_name = 'start_workflow', null = True, blank = True)
bodyfmt = models.ForeignKey(Format, related_name = 'start_workflow')
endfmt = models.ForeignKey(Format, related_name = 'start_workflow', null = True, blank = True)
So I need this model to validate that startfmt, bodyfmt and endfmt reference formats that share the same printer... how ?
Your best bet is probably overriding save in the Workflow model:
class Workflow(models.Model):
# field definitions as you have them
def save(self, force_insert=False, force_update=False):
printer = self.bodyfmt.printer
if self.startfmt and self.startfmt.printer != printer:
raise ValueError("Startfmt printer does not match")
if self.endfmt and self.endfmt.printer != printer:
raise ValueError("Endfmt printer does not match")
super(Workflow, self).save(force_insert, force_update)
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