Improved public_for_api scopes and corresponding specs

spec/models/comment_spec.rb

@@ -134,7 +134,7 @@ describe Comment do
     end
   end
 
-  describe "public_for_api" do
+  describe "public_for_api scope" do
     it "returns comments" do
       comment = create(:comment)
 
@@ -174,5 +174,12 @@ describe Comment do
 
       expect(Comment.public_for_api).not_to include(comment)
     end
+
+    it 'does not return comments on budget investments' do
+      budget_investment = create(:budget_investment)
+      comment = create(:comment, commentable: budget_investment)
+
+      expect(Comment.public_for_api).not_to include(comment)
+    end
   end
 end