Upsert
data class Upsert(val path: String, val key: String, val value: <Error class: unknown class>, val onConflict: <Error class: unknown class>? = null) : MutationAction
"Add it, or adjust the one that is already there." An existing match is merged with onConflict, or replaced by value outright when no onConflict is given; no match appends value. value must carry the key field, since that is what identifies it.