JSON Merge-Patch examples: A simple implementation of json merge-patch (http://tools.ietf.org/html/draft-ietf-appsawg-json-merge-patch/). Code to extract the examples from the draft helps see the difference that eliding purge_nulls makes. Because of recent changes to JSON (non-containers at top level), requires "oj" gem.

merge-patch-out.txt

Let's see where we don't agree (this code doesn't do purge_nulls):
Mismatch: 
orig:     {"a":"foo","b":[3,null,{"x":null}]}
patch:    {"b":[3,null,{"x":null}]}
draft:    {"a":"foo","b":[3,{}]}
actual:   {"a":"foo","b":[3,null,{"x":null}]}
Mismatch: 
orig:     [1,2]
patch:    [1,null,3]
draft:    [1,3]
actual:   [1,null,3]
Mismatch: 
orig:     [1,2]
patch:    [1,null,2]
draft:    [1,2]
actual:   [1,null,2]
Mismatch: 
orig:     {"a":[{"z":1,"b":null}]}
patch:    {"a":[{"z":1,"b":null}]}
draft:    {"a":[{"z":1}]}
actual:   {"a":[{"z":1,"b":null}]}
Mismatch: 
orig:     {"a":"foo"}
patch:    null
draft:    "Invalid Example"
actual:   null
Mismatch: 
orig:     {"a":"foo"}
patch:    "bar"
draft:    "Invalid Example"
actual:   "bar"

merge-patch.rb

def merge_patch(orig, patch)
  if Hash === patch
    orig = {} unless Hash === orig
    patch.each do |k, v|
      if v.nil?
        orig.delete(k)
      else
        orig[k] = merge_patch(orig[k], v)
      end
    end
    orig
  else
    patch
  end
end                             # yep, that's 15 lines.

data = DATA.read.lines.map(&:chomp)
cols = data.shift.scan(/\w*\s+/).map(&:size)
data.shift                      # discard ---

examples = [["", "", ""]]
data.each { |line|
  s = cols.map { |c| line.slice!(0, c) } << line
  if s.shift == ""
    examples << ["", "", ""]
  else
    examples.last.each_with_index do |ex, i|
      ex << s[i].strip
    end
  end
}

require 'oj'                    # this json does non-containers
triples = examples.map do |ex|
  ex.map {|s| Oj.load(s) rescue "Invalid Example"}
end

puts "Let's see where we don't agree (this code doesn't do purge_nulls):"
triples.each do |o, p, r|
  res = merge_patch(o, p)
  if r != res
    puts "Mismatch: "
    puts "orig:     #{Oj.dump(o)}"
    puts "patch:    #{Oj.dump(p)}"
    puts "draft:    #{Oj.dump(r)}"
    puts "actual:   #{Oj.dump(res)}"
  end
end

__END__
             ORIGINAL        PATCH           RESULT
             ------------------------------------------
             {"a":"b"}       {"a":"c"}       {"a":"c"}

             {"a":"b"}       {"b":"c"}       {"a":"b",
                                              "b":"c"}

             {"a":"b"}       {"a":null}      {}

             {"a":"b",       {"a":null}      {"b":"c"}
              "b":"c"}

             {"a":["b"]}     {"a":"c"}       {"a":"c"}

             {"a":"c"}       {"a":["b"]}     {"a":["b"]}

             {"a": {         {"a": {         {"a": {
               "b": "c"}       "b": "d",       "b": "d"
             }                 "c": null}      }
                             }               }

             {"a": [         {"a": [1]}      {"a": [1]}
               {"b":"c"}
              ]
             }

             ["a","b"]       ["c","d"]       ["c","d"]

             {"a":"b"}       ["c"]           ["c"]

             [1,2]           {"a":"b",       {"a":"b"}
                              "c":null}

             {"e":null}      {"a":1}         {"e":null,
                                              "a":1}

             {}              {"a": {         {"a": {
                               "bb": {         "bb": {
                                "ccc":null   }}}
                             }}}

             {"a":"foo"}     {"b": [         {"a":"foo",
                               3,             "b": [3, {}]
                               null,         }
                               {"x":null}
                              ]
                             }

             [1,2]           [1,null,3]      [1,3]

             [1,2]           [1,null,2]      [1,2]

             {"a":"b"}       {"a": [         {"a": [
                               {"z":1,         {"z":1}
                                "b":null      ]
                               }             }
                              ]
                             }

             {"a":"foo"}     null            Invalid Patch

             {"a":"foo"}     "bar"           Invalid Patch