Add SDG::Relation model

spec/models/sdg/relation_spec.rb

@@ -0,0 +1,33 @@
+require "rails_helper"
+
+describe SDG::Relation do
+  describe "Validations" do
+    it "is valid with a related SDG and a relatable model" do
+      relation = SDG::Relation.new(related_sdg: SDG::Goal[1], relatable: create(:proposal))
+
+      expect(relation).to be_valid
+    end
+
+    it "is not valid without a related SDG" do
+      relation = SDG::Relation.new(relatable: create(:proposal))
+
+      expect(relation).not_to be_valid
+    end
+
+    it "is not valid without a relatable model" do
+      relation = SDG::Relation.new(related_sdg: SDG::Goal[1])
+
+      expect(relation).not_to be_valid
+    end
+
+    it "is not valid when a relation already exists" do
+      proposal = create(:proposal)
+      goal = SDG::Goal[1]
+
+      SDG::Relation.create!(related_sdg: goal, relatable: proposal)
+      relation = SDG::Relation.new(related_sdg: goal, relatable: proposal)
+
+      expect(relation).not_to be_valid
+    end
+  end
+end