[Django]-Django Rest Framework foreign key nesting

class RelationshipSerializer(serializers.ModelSerializer):
    user = UserSerializer(read_only=True)
    related_user = UserSerializer(read_only=True)

    class Meta:
        model = models.Relationship
        fields = (
            'user',
            'related_user',
            'relationship'
        )

user = UserSerializer(read_only=True, many=True) is for manytomany field,user = UserSerializer(read_only=True) is for ForeignKey field.