[Zope3-Users] select field missing value formlib validation

[Dennis Schulz]

I would like to do a select field that updates also the no value in the 
DB when selected.
I already have a Schema Choice field that has required = False.
The (no value) with value="" value is displayed but when I try to apply 
there is the Validation Error Message Invalid Value.
I tried to set the _missing and missing_value attributes of the field.

I figured out that _missing is the rendered "value" of the missing entry 
and missing_value should be True but I still could not get it working 
with a litte playing on the parameters.
Maybe someone could explain a litte bit the meaning of these values?