class BSON::DBRef

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