How to model this complex validation for uniqueness on combined fields - ruby-on-rails

Question

A link has two components: componenta_id and componentb_id. To this end, in the Link model file I have:
belongs_to :componenta, class_name: "Component"
belongs_to :componentb, class_name: "Component"
validates :componenta_id, presence: true
validates :componentb_id, presence: true
validates :componenta_id, uniqueness: { scope: :componentb_id }
validates :componentb_id, uniqueness: { scope: :componenta_id }
And in the migration file:
create_table :links do |t|
t.integer :componenta_id, null: false
t.integer :componentb_id, null: false
...
end
add_index :links, :componenta_id
add_index :links, :componentb_id
add_index :links, [:componenta_id, :componentb_id], unique: true
Question: This all works. Now I want the combination of componanta and componentb to be unique, irrespective their order. So irrespective which component is componenta and which one is componentb (after all that's the same link; a link between the two same components). So the two records below should not be allowed since they represent the same link and thus are not unique:
componenta_id = 1 ; componentb_id = 2
componenta_id = 2 ; componentb_id = 1
How can I create this uniqueness validation? I have model validation working (see below) but wonder whether and how I should also add validation at the migration/db level...?
Model validation
I have model validation working with the code below:
before_save :order_links
validates :componenta_id, uniqueness: { scope: :componentb_id }
private
def order_links
if componenta_id > componentb_id
compb = componentb_id
compa = componenta_id
self.componenta_id = compb
self.componentb_id = compa
end
end
The following test confirms the above works:
1. test "combination of two links should be unique" do
2. assert #link1.valid?
3. assert #link2.valid?
4. #link1.componenta_id = 3 ##link2 already has combination 3-4
5. #link1.componentb_id = 4
6. assert_not #link1.valid?
7. #link1.componenta_id = 4
8. #link1.componentb_id = 3
9. assert_raises ActiveRecord::RecordNotUnique do
10. #link1.save
11. end
12.end
Migration/db validation:
As an extra level of security, is there also a way to incorporate validation for this at the db level? Otherwise it is still possible to write both of the following records to the database: componenta_id = 1 ; componentb_id = 2 as well as componenta_id = 2 ; componentb_id = 1.

Answer 1

Perhaps it is possible to control the creation of the links with:
def create_unique_link( comp_1, comp_2 )
if comp_1.id > comp_2.id
first_component = comp_1
second_component = comp_2
end
link = Link.find_or_create_by( componenta_id: first_comp.id, componentb_id: second_comp.id )
end
If you need the validation, then you can custom validate:
def ensure_uniqueness_of_link
if comp_1.id > comp_2.id
first_component = comp_1
second_component = comp_2
end
if Link.where( componenta_id: first_component.id, componentb_id: second_component ).first
errors.add( :link, 'Links should be unique' )
end
end

Answer 2

validates :componenta_id, uniqueness: { scope: :componentb_id }
validates :componentb_id, uniqueness: { scope: :componenta_id }