text: Q:
  
  Remove selection value from duplicate ForeignKey if already selected in the first
  
  I have the model Category:
  class Category(models.Model):
      name = models.CharField("Category Name", max_length=120, default='', null=True, blank=True)
  
  and the model Organization
  class Organization(models.Model):
      name = models.CharField("Organization name", max_length=120, default='')
      m_primary_category = models.ForeignKey('Category', null=True, blank=True, default='', related_name="+", verbose_name='Primary Category')
      m_secondary_category = models.ForeignKey('Category', null=True, blank=True, default='', related_name="+", verbose_name='Secondary Category')
  
  The user has to add categories first, then in Organization model, when selecting Primary & Secondary fields.
  The question is: how to hide/remove category from secondary, if the selection is already selected in primary? 
  Example: 
  - The user add: Cat1, Cat2, and Cat3 in Category Model. 
  - The user add: Organization name: Org, select Primary category: Cat1. then the available categories in "Secondary Category" must be: Cat2, Cat3. 
  
  A:
  
  If you wish to show and hide elements without submitting a form you'll need some javascript, using jquery this should be rather easy
  $('#id_m_primary_category').change(function(){
      // etc
  });
  
  Of course one cannot trust a user's input so you would still need to validate the form
  def clean(self):
      if self.cleaned_data.get('m_primary_category') == self.cleaned_data.get('m_secondary_category'):
          raise forms.ValidationError('No')
  
  