field with filenames.clj

(reduce (fn [m [k vs]]
          (into {} (map #(update-in m [%] conj k) vs)))
        {}
        {"filename" ["field1" "shared-field1" "field3"]
        "filename2" ["field4" "field5" "shared-field1"]
        "filename3" ["field9" "shared-field1"]})

=> {"field9" ("filename3"),
    "field4" ("filename2"),
    "field1" ("filename"),
    "shared-field1" ("filename3" "filename2" "filename"),
    "field3" ("filename"),
    "field5" ("filename2")}