title: django - get() returned more than one topic
body: <p>When I tried to relate an attribute with another one which has an <strong>M to M</strong> relation I received this error:</p>
  
  <blockquote>
    <p>get() returned more than one topic -- it returned 2!</p>
  </blockquote>
  
  <p>Can you guys tell me what that means and maybe tell me in advance how to avoid this error ?</p>
  
  <p><strong>models</strong></p>
  
  <pre><code>class LearningObjective(models.Model):
      learning_objective=models.TextField()
  
  class Topic(models.Model):
      learning_objective_topic=models.ManyToManyField(LearningObjective)
      topic=models.TextField()
  </code></pre>
  
  <p>output of <code>LearningObjective.objects.all()</code></p>
  
  <pre><code>[&lt;LearningObjective: lO1&gt;, &lt;LearningObjective: lO2&gt;, &lt;LearningObjective: lO3&gt;]
  </code></pre>
  
  <p>output of <code>Topic.objects.all()</code></p>
  
  <pre><code>[&lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;, &lt;Topic: Topic object&gt;]
  </code></pre>
  
  <p><strong>views</strong></p>
  
  <pre><code> def create_themen(request):
       new_topic=Topic(topic=request.POST['topic'])
       new_topic.save()
       return render(request, 'topic.html', {'topic': topic.objects.all()})
  
   def create_learning_objective(request):
       new_learning_objective=LearningObjective(learning_objective=request.POST['learning_objective'])
       new_learning_objective.save()
       new_learning_objective_topic=Topic.objects.get(topic=request.POST['topic'])
       new_learning_objective_topic.new_learning_objective_topic.add(new_learning_objective)
       return render( request, 'learning_objective.html', {
                      'topic': Topic.objects.all(),
                      'todo': TodoList.objects.all(),
                      'learning_objective': LearningObjective.objects.all()
                    })
  </code></pre>