web2py how to: Ajax to send dropdown menu's value to action - ajax

Question

I have a working dropdown menu, and I can send its value to a function using "submit" button. It's quite clumsy however, as user always has to press the button, wait for the page to load and it refreshes the page so the user loses all other "settings" made on the page. I have understood that ajax would be the solution for this. I read the guide: http://www.web2py.com/books/default/chapter/29/11/jquery-and-ajax#The-ajax-function and tried a few methods, but it never works.
So this is my original working code. Some contents are stripped and altered, but the basics are the same. View demo.html:
<form action="change_dropdown">
<select name="tables">
<option value="first_value">first</option>
<option value="second_value">second</option>
<option value="third_value">third</option>
</select>
<br><br>
<input type="submit">
</form>
Action:
def change_dropdown():
if request.vars.tables:
session.tables = request.vars.tables
else:
session.tables = None
session.main_warning= "Incorrect parameters for function: 'change_dropdown()'."
redirect(URL('demo'))
return
Then the original action demo does something with session.tables and so on. But now about turning it to ajax. This is what I want:
<form>
<select name="tables", onchange="ajax('change_dropdown', [], '');">
<option value="first_value">first</option>
<option value="second_value">second</option>
<option value="third_value">third</option>
</select>
</form>
I also did this to the action: redirect(URL('demo'), client_side=True) as mentioned in an example. I have no idea why it's needed however.
But I don't know how to send the variable tables to the action. If I write it inside python URL helper, it crashes, because it thinks it's a python variable (where it's actually a JavaScript variable (?)). If I write it inside ajax() function's second parameter, it halts and gives me a weird error in JS console:
Uncaught Error: Syntax error, unrecognized expression: [name=[object HTMLSelectElement]]
If you need more information I can show you full codes for the methods I tried, but I think someone can take it from here.

Answer 1

You can send the variable as follows:
ajax("{{=URL('change_dropdown')}}",['tables'],':eval')
redirect(URL('demo'), client_side=True) is needed as you want to redirect the function that have sent the ajax request to redirect not 'change_dropdown' to redirect.
EDIT:
<form>
<select name="tables", onchange="ajax(\"{{=URL('change_dropdown')}}\",['tables'],':eval') ;">
<option value="first_value">first</option>
<option value="second_value">second</option>
<option value="third_value">third</option>
</select>
</form>