uuid-fixtures.rb

# fixture
one:
  type: TextTimelineItem
  item_text: Hello, hello, happiness
  
# controller test
test "should create comment for a commentable" do
  t = text_timeline_items(:one)
  
  # t.id changes for every test :(
  
  post comments_url,
       params: { content: "Congratulations", "commentable_type": "TextTimelineItem", "commentable_id": t.id },
           as: :json
           
  assert_response :created
end

# schema.rb
create_table "comments", id: :uuid, default: -> { "gen_random_uuid()" } do
  t.uuid commentable_id
  t.text commentable_type
  t.text content
  ...
end