title: post_save signal for multiple related models
body: <p>I need to trigger any changes of some models. My models are: Brand, Product, Package. <code>Package</code> has <code>fk</code> to <code>Product</code> and Product has <code>fk</code> to <code>Brand</code>. So when some instances of these models are changed or created or deleted, I need to send signal. Can I implement it with <code>post_save</code> signal? I thought that if I write <code>post_save</code> signal for lower model: <code>Package</code>, then any changes with <code>Brand</code> or <code>Product</code> will be triggered. But it's not</p>