log.sql

  User Exists? (1.3ms)  SELECT 1 AS one FROM "users" WHERE "users"."username" = $1 LIMIT $2  [["username", "michael"], ["LIMIT", 1]]
  User Create (0.5ms)  INSERT INTO "users" ("username", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["username", "michael"], ["created_at", "2022-12-29 14:51:02.384023"], ["updated_at", "2022-12-29 14:51:02.384023"]]
  User Exists? (0.4ms)  SELECT 1 AS one FROM "users" WHERE "users"."username" = $1 LIMIT $2  [["username", "lebron"], ["LIMIT", 1]]
  User Create (0.2ms)  INSERT INTO "users" ("username", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["username", "lebron"], ["created_at", "2022-12-29 14:51:02.386506"], ["updated_at", "2022-12-29 14:51:02.386506"]]
  User Exists? (0.3ms)  SELECT 1 AS one FROM "users" WHERE "users"."username" = $1 LIMIT $2  [["username", "michael"], ["LIMIT", 1]]
  TRANSACTION (0.3ms)  ROLLBACK

user.rb

class User < ApplicationRecord
  validates_uniqueness_of :username
end

user_test.rb

require "test_helper"

class UserTest < ActiveSupport::TestCase
  test "inserting users in a transaction" do
    assert_difference ->{ User.count }, 0 do
      begin
        ActiveRecord::Base.transaction do
          User.create!(username: "michael") # short-cut for `new(...).save!`
          User.create!(username: "lebron")
          User.create!(username: "michael") # not unique error
        end
      rescue ActiveRecord::RecordInvalid
        nil
      end
    end
  end   
end