class Klass

  def initialize
    db = Sequel.connect('sqlite://example.db')
    @items = db[:subscriptions]
  end

# some code ...

  def check_for_duplicates(m)
    @items.filter(:mail => m).first[:mail] == m
  end
# some code ...
end

obj = Klass.new
obj.check_for_duplicates('example@gmail.com')

# can't express #<Klass:0xb772330c @items=#<Sequel::SQLite::Dataset: "SELECT * FROM `subscriptions`">> as a SQL literal