Multiple scope uniqueness validation

Question

I wanted to clarify the behavior of multi-scope uniqueness validation. The documentation says:

Or even multiple scope parameters. For example, making sure that a teacher can only be on the schedule once per semester for a particular class.

class TeacherSchedule < ActiveRecord::Base
  validates_uniqueness_of :teacher_id, :scope => [:semester_id, :class_id]
end

My understanding of this is that I could have a teacher teaching two classes in the same semester but not the same class, and I could have a teacher teaching the same class in different semesters. Is this correct? All 3 fields must match some existing record in order for validation to fail?

Is there a way to validate it so that it fails if either semester_id or class_id matches?

Answer 1

My understanding of this is that I could have a teacher teaching two classes in the same semester but not the same class, and I could have a teacher teaching the same class in different semesters. Is this correct? All 3 fields must match some existing record in order for validation to fail?

Yes, this is correct. Thing about it as "for every unique value of scope, the field can only show up once." When scope is an array, a "unique value for scope" is a combination of the fields' values.

Is there a way to validate it so that it fails if either semester_id or class_id matches?

So a teacher should never teach twice in a semester, and also should never ever teach the same class, even in a different semester? That doesn't seem right, but you could do that with a validation on each:

class TeacherSchedule < ActiveRecord::Base
  validates_uniqueness_of :teacher_id, :scope => :semester_id
  validates_uniqueness_of :teacher_id, :scope => :class_id
end