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(let [items [{:id 1 :name "primary"}
             {:id 2 :name "secondary" :parent {:id 1}}
             {:id 3 :name "secondary" :parent {:id 1}}
             {:id 4 :name "also primary"}]]
  (group-by #(:id (or (:parent %) %))
            items))
;=> {1 [{:id 1, :name "primary"}
;       {:id 2, :name "secondary", :parent {:id 1}}
;       {:id 3, :name "secondary", :parent {:id 1}}],
;    4 [{:id 4, :name "also primary"}]}