= Querying fields before merge =

Sometimes it can be really nice (read: improve the user experience) to be able to query some fields without merging the value from the form back into an object.  An example is when a user changes a input select field in a form which has an attached value.  In an app we have, a user has a select list of product numbers.  We make the app update the product's name on the page when the product changes.

Note that we are treading on thin ice here: if the user has not entered a valid value for the field, when {{{get_value}}} tries to convert it to the appropriate type, it will raise an exception that will need to be dealt with.  It is (generally) safe to reference values from a TextField or a SelectField though.

{{{#!python
class ProductFieldsetForm(FieldsetForm):

    products_menu = product_factory.all_products_menu()

    def __init__(self, product):
        self.product_id = product.product_id
        elements = (
            Fieldset((
                SelectField('Product code', self.products_menu, 'product_id',
                                 html_attrs={'onchange': 'javascript:product_form.submit()'}),
                StaticField('Product', product.name),
            )),
        )
        FieldsetForm.__init__(self, 'Product selection', elements)
        
        self.load(product)  

    def update(self):
        product_code_field = self.get_field('Product code')
        if self.product_id != product_code_field.get_value():
            self.product_id = product_code_field.get_value()
            product = product_factory.product_with_product_id(self.product_id)
            product_field = self.get_field('Product')
            product_field.set_value(product.name)
}}}

The interesting part here is where we query the form for a specific field using its {{{get_field}}} method.

We use the little bit of Javascript to make a change to the select list force an update to the page.  That means having to name the form when we render the page:

{{{#!html
<al-form method="post" name="product_form">

  <alx-form name="product_form_form" />

</al-form>
}}}

In the app itself, we use:

{{{#!python
def page_enter(ctx):
    if not ctx.has_value('product_form'):
        ctx.locals.product_form = ProductForm(ctx.locals.product)
        ctx.add_session_vars('product_form')

def page_process(ctx):
    ctx.locals.product_form.update()      # update product name if product changed
    if ctx.request('save'):
        [...]
}}}