Association of Associations : how to permute level 1 and level 2 keys?

I have a ragged Association of Association, say :

assoc = Association[
  "1" -> Association["a" -> "x", "b" -> "y"], 
  "2" -> Association[            "b" -> "z", "c" -> "k"]
                   ]  

I would like to transform it into a Association where level 1 and level 2 keys are reversed, that is to say :

Association[
      "a" -> Association["1" -> "x"], 
      "b" -> Association["1" -> "y", "2" -> "z"],
      "c" -> Association[            "2" -> "k"]
           ]

My solution is :

keysExplodedList = Reap[MapIndexed[Sow[Reverse[#2] -> #1] &, assoc, {2}]][[2, 1]]
groupedLevel1 = GroupBy[#[[1, 1]] &] @ keysExplodedList

groupedLevel2 = GroupBy[#[[1, 2]] &] /@ groupedLevel1
result = Map[#[[1, 2]] &, groupedLevel2, {2}]

<|"a" -> <|"1" -> "x"|>, "b" -> <|"1" -> "y", "2" -> "z"|>, "c" -> <|"2" -> "k"|>|>

Is there something more elegant ?

Answer

Mathematica 10.1 almost supports this operation directly:

assoc // Query[Transpose]


(*
<| "a" -> <|"1" -> "x", "2" -> Missing["KeyAbsent", "a"]|>,
   "b" -> <|"1" -> "y", "2" -> "z"|>,
   "c" -> <|"1" -> Missing["KeyAbsent", "c"], "2" -> "k"|>
|>
*)

All that remains is to delete the unwanted Missing elements:

assoc // Query[Transpose] // DeleteMissing[#, 2]&


(*
<| "a" -> <|"1" -> "x"|>,
   "b" -> <|"1" -> "y", "2" -> "z"|>, 
   "c" -> <|"2" -> "k"|>
|>
*)

We can see that Query uses the undocumented function GeneralUtilities`AssociationTranspose to do the heavy-lifting:

Query[Transpose] // Normal


(* GeneralUtilities`AssociationTranspose *)

assoc // GeneralUtilities`AssociationTranspose

(*
<| "a" -> <|"1" -> "x", "2" -> Missing["KeyAbsent", "a"]|>,
   "b" -> <|"1" -> "y", "2" -> "z"|>,
   "c" -> <|"1" -> Missing["KeyAbsent", "c"], "2" -> "k"|>
|>

*)

An Imperative Solution

The words "elegant" and "imperative" rarely appear together these days, but an imperative solution can express the transposition directly:

Module[{r = <| |>}
, Do[r = Merge[{r, <| j -> <| i -> assoc[[i, j]] |> |>}, Association]
  , {i, Keys[assoc]}
  , {j, Keys[assoc[[i]]]}
  ]
; r

]

(*
<| "a" -> <|"1" -> "x"|>,
   "b" -> <|"1" -> "y", "2" -> "z"|>, 
   "c" -> <|"2" -> "k"|>
|>
*)

A ScanIndexed operator would come in handy here (the undocumented one in GeneralUtilities` is not, well, general enough).