https://github.com/sferik/rails_admin/wiki/Associations-basics

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If you set the :inverse_of option on your relations, RailsAdmin will automatically populate the inverse relationship in the modal creation window. (link next to :belongs_to and :has_many multiselect widgets)

It will also hide the inverse relation on nested forms. As a good practice, you should always set :inverse_of options to all your associations, even if these are useless to ActiveRecord (in combination with :through and :as). RailsAdmin will take advantage of them.