Django: field's default value from self model's instances

Question

How can I make default value for a field to be taken from existing objects of a model?

I tried these and it didn't worked:

1)

class ModelA(models.Model):     fieldA = models.CharField(default=self.get_previous())      def get_previous(self):         return ModelA.objects.all()[0].fieldA 

NameError: name 'self' is not defined

2)

class ModelA(models.Model):     fieldA = models.CharField(default=ModelA.get_previous())      @staticmethod     def get_previous():         return ModelA.objects.all()[0].fieldA 

NameError: name 'ModelA' is not defined

3)

class ModelA(models.Model):     fieldA = models.CharField(default=get_previous())  def get_previous():     return ModelA.objects.all()[0].fieldA 

NameError: global name 'get_previous' is not defined

4)

def get_previous():     return ModelA.objects.all()[0].fieldA  class ModelA(models.Model):     fieldA = models.CharField(default=get_previous()) 

NameError: global name 'ModelA' is not defined

I it's clear why 3) and 4) won't work. I can imagine why 1) won't work - looks like class' properies can't refer to instance's (i.e. self). I can imagine why 2) won't work - apparently there's no reference to ModelA until interpreter will go trough whole class.

So how should I approach this?

Answer 1

In your examples, you need to remove the call operator ().

Currently the statement is executed immediately at the first read-parsing cycle. By specifying the symbol name instead, the Django class receives a function pointer which it will execute when it actually needs the default value.

The example becomes:

def get_previous():     return ModelA.objects.all()[0].fieldA  class ModelA(models.Model):     fieldA = models.CharField(default=get_previous) 

If you're going to do this for a lot of fields, consider overridding the save function, so you only have to fetch the previous object from the database just once.

Answer 2

If you want to just output default value, override __getattr__() method like this:

class ModelA(models.Model):   # your fields   def __getattr__(self, name):     if(name == 'fieldA' and !self.fieldA):       return self.get_previous()     else:       return super(ModelA, self).__getattr__(name) 

Saving default value from object will be little difficultier. First solution that comes in my mind, override save() method (i believe there is simpler solution)