class BSON::DBRef

def as_json(*args)
  {}.update(self)
end

def collection
  self['$ref']
end

def database
  self['$db']
end

def id
  self['$id']
end

def initialize(hash_or_collection, id = nil, database = nil)
  if hash_or_collection.is_a?(Hash)
    hash = hash_or_collection
    unless id.nil? && database.nil?
      raise Error::InvalidDBRefArgument, 'When using the hash API, DBRef constructor accepts only one argument'
    end
  else
    warn("BSON::DBRef constructor called with the legacy API - please use the hash API instead")
    if id.nil?
      raise Error::InvalidDBRefArgument, 'When using the legacy constructor API, id must be provided'
    end
    hash = {
      :$ref => hash_or_collection,
      :$id => id,
      :$db => database,
    }
  end
  hash = reorder_fields(hash)
  %w($ref $id).each do |key|
    unless hash[key]
      raise Error::InvalidDBRefArgument, "DBRef must have #{key}: #{hash}"
    end
  end
  unless hash['$ref'].is_a?(String)
    raise Error::InvalidDBRefArgument, "The value for key $ref must be a string, got: #{hash['$ref']}"
  end
  if db = hash['$db']
    unless db.is_a?(String)
      raise Error::InvalidDBRefArgument, "The value for key $db must be a string, got: #{hash['$db']}"
    end
  end
  super(hash)
end

def reorder_fields(hash)
  hash = BSON::Document.new(hash)
  reordered = {}
  reordered['$ref'] = hash.delete('$ref')
  reordered['$id'] = hash.delete('$id')
  if db = hash.delete('$db')
    reordered['$db'] = db
  end
  reordered.update(hash)
end

def to_bson(buffer = ByteBuffer.new)
  as_json.to_bson(buffer)
end