Returning two things in one HttpResponse

Question

I am trying to implement paging across ajax calls. The page should not refresh when the user wants to see the next x num of results.

Here is my problem. Returning the QuerySet is super simple. I just do (sumaJson is custom)

data = serializers.serialize('sumaJson', result_page.object_list, relations=('first_major', 'country_of_origin', 'second_major'))
return HttpResponse(data, mimetype="application/json") 

Now I also want to return things like

result_page.has_previous()
result_page.has_next()
result_page.paginator.count

and so on. I for the life of me can't figure out how to get both across in one response. I can't add this info to result_page.object_list because then the serializer fails. If I something of the sort of

simplejson.dumps(paging_info + result_page.object_list)

Then in the javascript the QuerySet is no longer a list of objects but just a big string of characters which can't be interpreted with

$.each(data.data, function(index, item){

I tried some bad hacks like creating a fake object and putting it in the object_list, serializing this and then deleting the object. This allows me to get the data across. However, I don't want to be creating and deleting fake objects.

I don't want to meddle with the serializer. I don't want to send a second ajax request once I get the querySet back to get the paging info.

Am I missing something? Is there an easy way to get both across in one response? Thanks!

Answer 1

What I've done in the past for ajax calls is I return the json as the HttpResponse, like you're doing, and add a header for any additional fields that I want to return.

data = serializers.serialize('sumaJson', result_page.object_list, relations=('first_major', 'country_of_origin', 'second_major'))
response = HttpResponse(data, mimetype="application/json") 
response['X-VALUE'] = 'asdf' #this is the header, you can create as many of these as you'd like
return response

and on the javascript side...

$.ajax({
  url: '/whatever/here/',
  success: function(data, code, xhr) {
    alert(xhr.getResponseHeader('X-VALUE'));
  }
});

hope this helps.

Answer 2

When I'm serializing a collection of objects, I typically include the pagination information in the response body itself. If I had 50 objects that I wanted to serve up 10 per page, the JSON would look something like this:

{
    "count": 50,
    "objects": [
        {
            ...
        }
    ],
    "pages": {
        "count": 5,
        "current": "http://api.example.com/objects/?page=3",
        "first": "http://api.example.com/objects/",
        "last": "http://api.example.com/objects/?page=5",
        "next": "http://api.example.com/objects/?page=4",
        "previous": "http://api.example.com/objects/?page=2"
    }
}

Answer 3

simplejson.dumps() can serialize dictionaries deeply, even recursively.

There are two approaches you can take to this. The first is to use the Django ORM's values() method on QuerySets: it churns out pure python dictionaries with object IDs instead of references, suitable for serializing.

If you need even deeper than that, you may have to write something to create the dictionary structure proposed by jpwatts. If you need that kind of power, I have a post on my personal blog about adding functors, generators, iterators, and closures to simplejson. The example demonstrates how to turn a Treebeard tree structure into a javascript object.

The code is:

from django.utils.simplejson.encoder import JSONEncoder

class ExtJsonEncoder(JSONEncoder):
    def default(self, c):
        # Handles generators and iterators
        if hasattr(c, '__iter__'):
            return [i for i in c]

        # Handles closures and functors
        if hasattr(c, '__call__'):
            return c()

        return JSONEncoder.default(self, c)

jpwatts has the correct approach. You may just have to write some code to get there.