Thanks @Hilar for you answer, I tried to comment you answer directly, but I don't have enough Karma. (By the way, this Karma thing is bullshit!)

Your answer gave me some light on this matter, though I'm having the same syntax error.

I also tried this: 

Since the x_order_prirority is a selection, so I wrote the code like this: 

for record in self: 
    if record.x_order_prirority.selection_ids.value = '3' :  // selection_ids.value is actually a 'char' type, not an integer
        record.x_rush = True

But again, syntax error.

Any help? Thanks.

self will be the current model object. you must assign values as

self.field_name = x or self['field_name'] = x

In your case it must be

for record in self:
  if record ['x_order_prirority'] = 3 :
    record.x_rush = True