Django Newbie ManyRelated Manager not Iterable Question

Question

Hello,

I'm trying to create a product code (in the admin) by combining elements from two other fields - one of which is a ManyToManyField. I'd like to iterate through that field to find out if a specific product option has been chosen, and append a variation of it to that non-editable product code, like so:

class ShirtColorClass(models.Model):
    shirtcolor = models.CharField(_('Shirt Color'), unique=True, max_length=40)
    def __unicode__(self):
        return self.shirtcolor

class ShirtClass(models.Model):
    shirtmodel = models.CharField(_('Model of Shirt'), max_length=40)
    shirtclr = models.ManyToManyField(_(ShirtColorClass, verbose_name='Shirt Color'))
    shirtcode = models.CharField(_('Code for the shirt'), max_length=80, editable=False)
    #...10 more fields...
    def __unicode__(self):
        return self.shirtmodel
    def save(self):
        for item in self.shirtclr: #these are the lines I'm not sure how to do
            if 'Blue' in self.shirtclr:
                self.shirtcode = u'%s%s' % ('B', self.shirtmodel)
            else:
                self.shirtcode = self.shirtmodel
            super(ShirtClass,self).save()

At the moment I'm getting a ManyRelatedManager not Iterable message, so I know I'm doing something wrong, but I don't know what... I apologize in advance for this being a stupid newbie question. Thank you.

Answer 1

Try calling .all() on it.

Answer 2

call filter():

def save(self):
    if self.pk!=None:
        if self.shirtclr.filter(shirtcolor='Blue'):
            self.shirtcode = u'%s%s' % ('B', self.shirtmodel)
        else:
            self.shirtcode = self.shirtmodel
    else:
        self.shirtcode = ''

you can avoid self.shirtcode = '' by adding default='' into

shirtcode = models.CharField(_('Code for the shirt'), max_length=80, editable=False, default='')

Answer 3

Thank you gentlemen for both your answers. I merged both of your answers into one...a property with a filter like so:

 def _get_blue_shirts(self):
  if self.shirtclr.filter(shirtcolor='Blue'):
   return '%s%s' % ('B', self.shirtmodel)
  else:
   return self.shirtmodel
 blue_shirts=property(_get_blue_shirts)

While this approach definitely works, I can see issues with it as I have written it. First, I would like to search in the admin with, for example, "B13A" as a shirt model and have it understand that what I mean is a "13A" shirt available with Blue as one of its colors. Since the ModelAdmin.search_fields seems to need to resolve to an actual field, this approach doesn't work in this regard (correct me if I'm wrong there). The other concern I have is the property seems very SQL-heavy...executing a separate select for each row (whereas a dedicated column would just need an overall select...again, correct me if I'm wrong). Any ideas as to how I might go about addressing these concerns? Perhaps another way I could go about it?

PS. Antony...I tried the revised overridden save() and still got the "'ShirtClass' instance needs to have a primary key value before a many-to-many relationship can be used" error. Am I doing something wrong here?

Many, Many, Many thanks to all, -bkev