I’ve been working on a project which is a
Rails Engine and recently
struggled through getting the engine’s asserts working within an app on Heroku.
Because of how Heroku sets up the environment for the asset precompiliation
step (or doesn’t set up the environment, as it happens), they recommend disabling the
initialize_on_precompile setting (many apps are unable to complete their normal initialization without their Heroku environment config, which isn’t available during Heroku’s build process;
turning off initialization during precompilation avoids that).
Once you do that, however, asset precompilation won’t trigger your Rails engine’s initialization, so even if you’ve set up your engine’s assets for precompilation as described in the
Rails Engine Guide, your assets won’t actually get precompiled. Why? Because when
initialize_on_precompile is disabled, only
:assets initializers are run during precompliation.
The trick is to get Rails to run your engine’s initializer during asset
precompilation even though it’s not actually initializing your whole
application. The magic incantation for that is to pass a special
:group => :assets or
:group => :all option to your engine’s initializer, and make
sure you configure your precompiled in that initializer.
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I first got this working by specifying
:group => :assets, but it turns out
:assets is only used during asset precompilation when
initialize_on_precompile is explicitly disabled.
So if you want your engine’s asset precompilation to work on other platforms
as well as Heroku you should use
:group => :all (which is special-cased to