HTTP PATCH: Handling arrays, deletion, and nested

I'm looking for a practical guide to implementing the PATCH verb for partial updates of a noun in a RESTful api using JSON. Understanding that PATCH is for partial updates, we lack still standardization around the syntax for deleting keys, creating or updating nested keys, and arrays.

Let's say I GET an object:

// GET users/42
{
  id: 42,
  name: 'SimpleAsCouldBe',
  city: 'San Francisco',
  roles: ['viewer','editor'],
  posts: {
    '01': {},
    '02': {},
  }
}

...Then I want to update it:

// PATCH users/42
{
  name: 'SimpleGuy',                   // CLEAR:   update the key's value
  email: 'hey@google.com',             // CLEAR:   add the new key
  city: null                           // UNCLEAR: delete the key?
  roles: ['owner'],                    // UNCLEAR: replace the whole array?
  posts: {
    '02': { title:'how to pop lock' }, // CLEAR:  update nested key
    '03': { title:'how to salsa' }     // CLEAR:  create new nested key
  }
  notes: {
    '01': { title: 'a note title' }    // CLEAR (but disallowed?): create wrapping key
  }
}

The PATCH rfc says no to creating nested keys. This is a spec bug, I think, because creating a nested key is non-ambiguous.

I could send a full object diff, like this library generates, but this makes the clear case of adding or updating a key more verbose.

How do I handle arrays, deletion, and nested keys in a lean way with HTTP PATCH?

The spec clearly details how to format the JSON body of a PATCH request. You're using a totally different format. Given that, I'm not surprised at all that there is ambiguity. The body should look something like:

   [
     { "op": "replace", "path": "/name", "value": "SimpleGuy" },
     { "op": "add", "path": "/email", "value": "hey@google.com" },
     { "op": "replace", "path": "/city", "value": null },
     { "op": "replace", "path": "/roles", "value": [ "owner" ] },
     { "op": "add", "path": "/posts/02/title", "value": "how to pop lock" },
     { "op": "add", "path": "/posts/", "value": "03" },
     { "op": "add", "path": "/posts/03/title", "value": "how to salsa" },
     { "op": "add", "path": "/notes", "value": { "title": "a note title" } }
   ]

Go back and read the spec. It even gives examples which address most of your questions.