jQuery Grid Plugin - jqGrid

Hello,

I feel a little bit lost using jqgrid to save data modified with an edit form.

I work with joomla. I'm a beginner with jquery, ajax (that's why I'm lost !!). Despite of reading a lot of web pages about the topic, I don't know how to process ...I hope my english is not to bad and that you will understand my need.

So, my problem is :

I have a grid, I select a row, a form is displayed with some of the fields + 2 new ones => that's ok.1

What I want to do on the form is :

Enter a number of credits in field 1 + debit (or credit) in field 2.

When I click on save, I want to :

1) Calculate the new value for credits (add/subtract field 1 to the field 'previous credits')

2) Update the DB with this new value for the selected row.

3) Insert a row in another table (log) to track the operation.

4) Display a message (ok/nok) + Refresh the grid with the new credit value

I don't know how to start...

What function should I have to invoke first ?

Should the keyword 'editurl' be equal to the path/name of my model which contains the update order ? If the insert order is in another model, how to do ? How to call the function update_table and send the data needed to these models ?

The update will be done via ajax, how to do to make it secure ?

If someone had a full example to share (or links), it will be nice !

Thanks a lot for you help

Thierry