module ActiveModel::Validations

def errors_on(attribute)

object.
Calls model.valid? in order to prepare the object's errors
Extension to enhance should have on AR Model instances.

model.should have(n).errors_on(:attribute)
model.should have(1).error_on(:attribute)
model.should have(:no).errors_on(:attribute)
:call-seq: 
def errors_on(attribute)
  self.valid?
  [self.errors[attribute]].flatten.compact
end