Get an array of object IDs returned from REST

[dmahaffey]

I get the following from a REST API call:

{
  "assignments": {
    "6c494931-6457-4faa-b893-7d7e985c9cc4": [
      19198153
    ],
    "9debce1c-c1a7-4742-80b6-e0e1e5cb3eec": [
      15405429,
      17840990,
      18936163
    ],
    "35f18ad1-d411-44b4-a211-8c7045d287f4": [
      17840990,
      19402036,
      22092514,
      20649120
    ]
  }
}

Each query returns an "assignments" object containing a variable number of unique object IDs (these are user IDs) and a variable number of integers in an array (these are file IDs) assigned to each user.

I need to count the number of user IDs, the number of file IDs for each user, and the number of file IDs for the set of users, but because the user IDs are objects, I can't figure out how to parse/select/loop the data to get the structure I need. I'd welcome any suggestions.