<collection-input>
This tag is called by <input> when the context is a has_many :through collection. By default a <select-many> is used, but this can be customised on a per-type basis. For example, say you would like the <check-many> tag used to edit collections a Category model in your application:
<def tag="collection-input" for="Category"><check-many merge/></def><collection-input for='ActiveRecord::Base'>
The default <collection-input> - calls <select-many>