are you using function field? provide field details.

no its not a functional field , its a readonly char field that takes its value in the create function

If you want to update the readonly field while editing record, do the same for write method.

Like Nehal say, openerp dont return readonly field, they assume readonly cant not be modify (my opinion is BAD IDEA), so i think you change the field with on_change function, but save dont pass the new value. You need to do what Nehal tell you, overide the write function and redo you calcul for this field. OR change the web client fot client send all field.