JSON problems in update method (ExtJs, Ext)

Hello rubyonrails peeps, my first post here.

I’ve been having a lot of trouble today getting my controller to
respond properly to an update request via json, coming from an
EditorGridPanel in ExtJS. I started off by using the ExtScaffold
plugin (which is great but limited in its examples), and hacked together
a helper for Ext’s EditorGridPanel.

I’ve tried both the json_request plugin and the built in rails 2.1
json handling. Both end up with the same problem:

NoMethodError (undefined method `{“id”:6, … }=’ for
#ModelName:0xb5957c74): etc…

So the JSON is not being parsed out as a nice neat params hash. I’m at
the point where I’m about to write my own parser or call the
json_request method directly to try and get this working. I’ve been
checking my json requests in Firebug and it seems to be ok, with the
required ‘application/json’ content-type:

Accept: application/json
Content-Type: application/json
authenticity_token: bcc54a2ffef408f8b0829cf310cabd411311fb92
… }

The only difference my json seems to have from the examples given in
both json_request examples and Rails
2.1 examples
) is that the format is
post: {“id”:4,“model_name[field_name]”:2,“model_name[field_name]”:“text”

rather than post: {“id”:4,field_name:2,field_name:“text” … }
(with the Rails 2.1 version ‘post’ is replaced by the model name)

my controller looks like this:

def update
respond_to do |format|
format.ext_json { render :json =>
this.to_ext_json(:success => true) }
logger.debug(“update failed!”)
format.ext_json { render :json =>
this.to_ext_json(:success => false) }
I’ve tried everything I can think of … it seems like I’m missing
something to trigger the json parsing, but I just don’t know what at
the moment.

Any help or ideas much appreciated!!