Make your data layer an implementation detail. :)

ar_factory_test.rb

require 'test_helper'

class ARFactory
  def self.new(table_name)
    Class.new(ActiveRecord::Base) do
      self.table_name = table_name
    end
  end
end

class User # look mom, no < ActiveRecord::Base! yay proper encapsulation
  @@datastore = ARFactory.new('test_users')

  def initialize
    @user = @@datastore.new
  end

  def name=(val)
    @user.name = val
  end

  def save
    @user.save
  end

  def self.find_by_name(string)
    @@datastore.find_by_name(string)
  end
end

class ARFactoryTest < ActiveSupport::TestCase
  def setup
    ActiveRecord::Migration.create_table('test_users') do |t|
      t.column 'name', :string
    end
    @user = User.new
  end

  def teardown
    ActiveRecord::Migration.drop_table('test_users')
  end

  def test_can_save_and_fetch_user
    @user.name = 'steve'
    @user.save
    @user = User.find_by_name('steve')
    assert_equal 'steve', @user.name
  end

  def test_test_user_not_defined
    assert !defined?(TestUser)
  end
end