feat(tube-resource): adds racked_tube relationship to v2 tube api

app/resources/api/v2/tube_resource.rb

@@ -46,6 +46,10 @@ def sibling_tubes
       # @!attribute [r] transfer_requests_as_target
       #   @return [Array<Api::V2::TransferRequestResource>] An array of transfer requests into this tube.
       has_many :transfer_requests_as_target, readonly: true
+
+      # @!attribute [r] racked_tube
+      #   @return [Api::V2::RackedTubeResource] The racked tube association.
+      has_one :racked_tube, readonly: true, foreign_key_on: :related
     end
   end
 end

spec/requests/api/v2/tubes_spec.rb

@@ -68,6 +68,10 @@
           expect(json.dig('data', 'relationships', 'transfer_requests_as_target')).to be_present
         end
 
+        it 'returns a reference to the racked_tube relationship' do
+          expect(json.dig('data', 'relationships', 'racked_tube')).to be_present
+        end
+
         it 'does not include attributes for related resources' do
           expect(json['included']).not_to be_present
         end

spec/resources/api/v2/tube_resource_spec.rb

@@ -19,6 +19,7 @@
   it { is_expected.to have_a_readonly_has_many(:aliquots).with_class_name('Aliquot') }
   it { is_expected.to have_a_readonly_has_one(:receptacle).with_class_name('Receptacle') }
   it { is_expected.to have_a_readonly_has_many(:transfer_requests_as_target).with_class_name('TransferRequest') }
+  it { is_expected.to have_a_readonly_has_many(:racked_tube).with_class_name('RackedTube') }
 
   # Behaviours
   it_behaves_like 'a labware resource'