module ActiveModel::Validations

def errors_on(attribute)

model.errors_on(:attribute).should include("can't be blank")

model.should have(n).errors_on(:attribute)
model.should have(1).error_on(:attribute)
model.should have(:no).errors_on(:attribute)

@example

You can also use this to specify the content of the error messages.

model.valid? in order to prepare the object's errors object.
Extension to enhance `should have` on AR Model instances. Calls 
def errors_on(attribute)
  self.valid?
  [self.errors[attribute]].flatten.compact
end