title: Set value of excluded field in django ModelForm programmatically
body: <p>I have a <code>ModelForm</code> class where I want to exclude one of the fields (<code>language</code>) from the form and add it programmatically. I tried to add the language in the clean method, but got an <code>IntegrityError</code> (<code>"Column 'language_id' cannot be null"</code>). I assume this is because any data for <code>language</code> returned from <code>clean()</code> will be ignored since it's in the exclude tuple. I'm using the form in a <code>ModelAdmin</code> class. I haven't managed to find any way to deal with this, so any tips or hints will be much appreciated.</p>
  
  <pre><code>from django import forms
  from myapp import models
  from django.contrib import admin
  
  
  class DefaultLanguagePageForm(forms.ModelForm):
      def clean(self):
          cleaned_data = super(DefaultLanguagePageForm, self).clean()
          english = models.Language.objects.get(code="en")
          cleaned_data['language'] = english
  
          return cleaned_data
  
      class Meta:
          model = models.Page
          exclude = ("language",)
  
  class PageAdmin(admin.ModelAdmin):
      form = DefaultLanguagePageForm
  </code></pre>