django rest framework - request context key error

I have two models (Like and News). I am using django-rest-framework to make a web api out of it.

class Like(models.Model):
    user = models.ForeignKey(User)
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey('content_type', 'object_id')

class News(models.Model):
    user = models.ForeignKey(User)
    title = models.CharField(max_length=150)
    ...
    likes = GenericRelation(Like)

A Like object has its own user field to store who liked the News object. Now to check if a specific user exists in any of the likes of a News object, I am getting request.user from a SerializerMethodField.

class NewsSerializer(serializers.HyperlinkedModelSerializer):
    user = UserSerializer()
    likes_count = serializers.IntegerField(source='likes.count', read_only=True)
    user_in_likes = serializers.SerializerMethodField()

    class Meta:
        model = News
        fields = ('user', 'title', 'body', 'article_image', 'pub_date', 'likes_count', 'user_in_likes')

    def get_user_in_likes(self, obj):
        user = self.context['request'].user
        what = obj.likes.filter(user=user).exists()
        return what

When I go the /news/ url, I get the json objects including the user_in_likes to true/false for each news object.

However, I have another serialzer for different model which imports NewsSerializer class and other similar serializers:

class ActivityObjectRelatedField(serializers.RelatedField):
    def to_representation(self, value):
        if isinstance(value, User):
            serializer = UserSerializer(value)
        elif isinstance(value, Job):
            serializer = JobSerializer(value)
        elif isinstance(value, News):
            serializer = NewsSerializer(value)
        elif isinstance(value, Tender):
            serializer = TenderSerializer(value)
        else:
            raise Exception('Unexpected type of tagged object')
        return serializer.data    

class ActivitySerializer(serializers.HyperlinkedModelSerializer):
    actor = ActivityObjectRelatedField(read_only=True)
    target = ActivityObjectRelatedField(read_only=True)

    class Meta:
        model = Activity
        fields = ('url', 'actor', 'verb', 'target', 'pub_date')

Now if I visit /activities/, to get the activities objects I am getting an error:

KeyError at /activities/

'request'

And it points to the line of SerializerMethod of NewsSerialize class where self.context['request'].user is used.

Exception Location: /vagrant/myproject/news/serializers.py in get_user_in_likes, line 25

Again if I visit /news/ url, everything is fine and I get news objects. What am I missing here? Why is request not being recognized in the ActivitiesSerializer class? Please help me solve this problem. Thank you.

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