schema = {
    "type" => "object",
    "required" => ["a","b"],
    "properties" => {
      "a" => {"type" => "integer"},
      "b" => {"type" => "string"},
      "c" => {
        "type" => "object",
        "properties" => {
          "z" => {"type" => "integer"}
        }
      }
    }
  }

  data = {
    "z" => 1
  }

  JSON::Validator.validate!(schema, data, :fragment => "#/properties/c")

Code almost from README, throws "Invalid schema encountered when resolving :fragment option (JSON::Schema::SchemaError)"

PS: In fact, there seem to be many problems with my test case, which is just one long schema with several object in it, like in http://stackoverflow.com/a/9083830. I'm not sure, what number of issues should be created (or what number of changes should I do in my code).

For example, having such schema (with "id"s and "$ref"s) in hash, I can't even start using it, receiving "`rescue in merge': both URI are relative (URI::BadURIError)" at the very beginning.