title: 'RelatedManager' object is not subscriptable
body: <p>thanks for your time.
  i'm trying to get the first images of a foreignKey image model and display with the other fields</p>
  
  <p>i've read some questions and docs about related models and the best i got was to create a function on my models to call it after just to get the first image.</p>
  
  <p>models.py:</p>
  
  <pre><code>class Veiculos (models.Model):
      YEAR_CHOICES = []
      for r in range(1960, (datetime.now().year+1)):
          YEAR_CHOICES.append((r, r))
  
      modelo = models.CharField(max_length=100)
      potencia = models.CharField(max_length=40)
      cor = models.CharField(max_length=30)
      preco = models.DecimalField(max_digits=8, decimal_places=2)
      ano = models.IntegerField(('ano'), choices=YEAR_CHOICES, default=datetime.now().year)
      category = models.ManyToManyField('Categorias')
      created_time = models.DateTimeField(auto_now=True)
      updated_time = models.DateTimeField(auto_now_add=True)
  
      def __str__(self):
          return '%s %s' % (self.modelo, self.preco)
  
      def get_absolute_url(self):
          return reverse('category2')
  
      def first_image(self):
          return self.images.first()  
  
  def get_image_filename(instance, filename):
      modelo = instance.veicle.modelo
      slug = slugify(modelo)
      return "veiculos_imagens/%s-%s" % (slug, filename)
  
  
  class Imagens (models.Model):
      veicle = models.ForeignKey(Veiculos, default=None, on_delete=models.CASCADE, related_name='images')
      imagem = models.ImageField(upload_to=get_image_filename)
  </code></pre>
  
  <p>views.py:</p>
  
  <pre><code>def amp_category(request):
      queryset = Veiculos.objects.all()
      return render(request, 'amp/category.amp.html', {'veiculos': queryset})
  </code></pre>
  
  <p>category.amp.html:</p>
  
  <pre><code>{% extends "amp/base.amp.html" %} {% block tittle %}
  &lt;title&gt;ok&lt;/title&gt;{% endblock tittle %} {% block content %}
  
  &lt;body&gt;
      &lt;h1&gt;ok2&lt;/h1&gt;
      {% for veiculo in veiculos %}
      &lt;h2&gt;{{veiculo.modelo}}&lt;/h2&gt;
      &lt;amp-img src="{{ veiculo.first_image.url }}" alt="ok" width="300" height="340"&gt;&lt;/amp-img&gt;
      {% endfor %}
  
  &lt;/body&gt;
  
  {% endblock %}
  
  &lt;/html&gt;
  
  </code></pre>
  
  <p>i'm getting the queryset objects although none imaged displayed
  <a href="https://github.com/lucasrf27/dealership" rel="nofollow noreferrer">https://github.com/lucasrf27/dealership</a></p>