Cannot set values on a ManyToManyField which specifies an in

#models.py
class Page(MPTTModel):
    title = models.CharField(max_length=255, verbose_name=_('Title'), help_text=_('Page\'s title.'))
class Style(models.Model):
    title = models.CharField(max_length=255, verbose_name=_('Title'))
    pages = models.ManyToManyField(Page, through='StyleRelationship')
class StyleRelationship(models.Model):
    page = models.ForeignKey(Page, verbose_name=_('Page'))
    style = models.ForeignKey(Style, verbose_name=_('Style'))
    ordering = models.IntegerField(default=0, verbose_name=_('Ordering'))
#forms.py
class StyleForm(forms.ModelForm):
    class Meta:
        model = Style

#views.py
def new_style(request, project_id):
    try:
        project = Project.objects.get(pk = project_id)
    except Project.DoesNotExist:
        raise Http404

    if request.method == 'POST':
        style_form = StyleForm(request.POST)
        if style_form.is_valid():
            style = style_form.save()
            return redirect('style', project_id, style.pk)
    else:
        style_form  = StyleForm()

    context = {
        'project': project,
        'projects_for_navigation': Project.objects.all(),
        'styles_for_navigation': Style.objects.all().filter(project = project),
        'style_form': style_form,
    }
    template = 'admin/styles/new_style.html'
    return render_to_response(template, context, context_instance=RequestContext(request))

#new_style.html
    <form action="" method="POST">
        <fieldset>
            <legend>{% trans 'New Style' %}</legend>
            {% csrf_token %}
            {% for field in style_form %}
                <p>{{ field.errors }}</p>
            {% endfor %}

            <input type="hidden" name="project" value="{{ project.pk }}">
            <p><label for="id_title">{{ style_form.title.label }}:</label> {{ style_form.title }}</p>
            <p><label for="id_code">{{ style_form.code.label }}:</label> {{ style_form.code }}</p>
            <p><label for="id_pages">{{ style_form.pages.label }}:</label> {{ style_form.pages }}</p>
        </fieldset>
        <input type="submit" value="{% trans 'Save' %}">
    </form>