Rails asset pipeline - image_path helper only working in development

Question

I have a problem with asset preocompilation in Rails (3.2.7).

I am including an favicon like this:

Answer 1

Ok, after a couple of tries I figured out, how to fix this. Nevertheless it´s a little bit strange and does not satisfy me completely. It only worked for me, when I set digest to true and provide the path to the manifest:

config.assets.compile = false
config.assets.digest = true
config.assets.manifest = Rails.root.join("public/assets")

It would be interesting to know, what´s behind this "logic".