;; next/genesis/projections/by-object.sx
;;
;; Index of activities that reference each :object CID. Maps
;; object-CID to the list of activity CIDs that target it
;; (Update / Delete / Announce / etc.). Used for "show me
;; everything that happened to X" queries.

(DefineProjection
  :name "by-object"
  :doc "Object CID -> list of activity CIDs that target it."
  :initial-state {}
  :fold (fn
    (state act)
    (let
      ((obj-cid (-> act :object)) (cid (-> act :cid)))
      (if
        (string? obj-cid)
        (assoc
          state
          obj-cid
          (append (or (get state obj-cid) (list)) (list cid)))
        state))))