title: django rest framework get() returned more than one object though filter() method is used
body: <p>In <code>model.py</code>, I've two models</p>
  
  <pre><code>class student(models.Model):
      name = models.CharField(max_length=255)
      registration_number = models.CharField(max_length=255)
      session = models.CharField(max_length=255)
      college = models.ForeignKey(to='college', on_delete=models.CASCADE)
  
      class Meta:
          db_table = 'student'
  
  
  class semester(models.Model):
      semester = models.IntegerField()
      student = models.ForeignKey(to='student', on_delete=models.CASCADE)
      sgpa = models.FloatField()
      grade = models.CharField(max_length=255)
      is_fail = models.BooleanField(default=True)
  
      class Meta:
          db_table = 'semester'
  </code></pre>
  
  <p>Serializer classes looks like</p>
  
  <pre><code>class SemesterSerializer(serializers.ModelSerializer):
      class Meta:
          model = models.semester
          fields = ('id', 'semester', 'sgpa', 'grade', 'is_fail',)
  
  class StudentSerializer(serializers.ModelSerializer):
      class Meta:
          model = models.student
          fields = ('id', 'name', 'registration_number', 'college', 'session',)
  </code></pre>
  
  <p>and <code>views.py</code> contains</p>
  
  <pre><code>class SemesterViewSet(viewsets.ModelViewSet):    
      def get_queryset(self):
          if 'semester' in self.kwargs:
              return models.semester.objects.filter(semester=self.kwargs['semester'])
          else:
              return models.semester.objects.all()
  
      lookup_field = 'semester'
      serializer_class = serializer.SemesterSerializer
  </code></pre>
  
  <p>url patterns are</p>
  
  <pre><code>router = DefaultRouter()
  router.register('semester', views.SemesterViewSet, base_name='semester')
  urlpatterns = [
      url(r'', include(router.urls)),
  ]
  </code></pre>
  
  <p>When I tried to visit <code>url/semester/8</code>, <code>MultipleObjectsReturned</code> exception raised and showes <code>get() returned more than one semester -- it returned 2!</code> error message. Here I've not used any <code>get()</code> method. Why this is happening and what is solution? I need to show all result of semester.</p>