Map Values in 'nested' association

Question

JSON often returns sort of 'nested' associations like so:

assoc = <|"key1" -> <|"a" -> 1, "b" -> 2|>, 
          "key2" -> <|"a" -> 3, "b" -> 4|>|>

It is often needed to apply f as follows.

assoc = <|"key1" -> <|"a" -> f[1], "b" -> 2|>, 
          "key2" -> <|"a" -> f[3], "b" -> 4|>|>

I see some traditional ways, i.e. MapAt, but I'm searching more specific way for the task, like KeyValueMap or something similar. Which one would you recommend? (Parsing APIs results is a good example of usecase.)

Answer 1

I have failed to find a good duplicate so:

assoc // Query[All, {"a" -> f}]

or

MapAt[f, assoc, {All, "a"}]

Answer 2

assoc = <|"key1" -> <|"a" -> 1, "b" -> 2|>, "key2" -> <|"a" -> 3, "b" -> 4|>|>;
assoc[[All, "a"]] = Map[f, Values[assoc[[All, "a"]]]];
assoc

<|"key1" -> <|"a" -> f[1], "b" -> 2|>, "key2" -> <|"a" -> f[3], "b" -> 4|>|>