An attempt with Sequel's composition plugin

record.rb

# In-memory test database
# The schema has been simplified for the discussion (yes, it should have some domain_id somewhere !)
#
# Values in the content field for a SOA, are, in order :
# ns email serial refresh retry expiry minimum
DB = Sequel.sqlite
DB.run "CREATE TABLE records (name VARCHAR(255) PRIMARY KEY NOT NULL, type VARCHAR(5), content VARCHAR(255))"
DB.run "INSERT INTO records VALUES ('example.com', 'SOA', 'ns.example.com root.example.com 2012333335 28800 86400 3600000 86400')"

# Just an assert function for basic testing
def assert
  printf '.'
  raise "#{caller} => assertion failed !" unless yield
end

# The record model
class Record < Sequel::Model
  plugin :composition

  # This is the list of the inner fields, located in the 'content' 
  # column of the SOA type records
  # The fields are listed in order so we can map easily to the 
  # splited column
  SOA_COLUMNS= [ :ns, :email, :serial, :refresh,
    :retry, :expiry, :minimum ]

  SOA = Struct.new(*SOA_COLUMNS)
  SOA_COMPOSER = proc{SOA.new(*content.split)}
  SOA_DECOMPOSER = proc{self.content = SOA_COLUMNS.map{|c| send(c)}.join(' ')}
  SOA_COLUMNS.each do |c|
    define_method(c){soa.send(c)}
    e = :"#{c}="
    define_method(e){|v| soa.send(e, v)}
  end

  composition :soa,
      :composer => SOA_COMPOSER,
      :decomposer => SOA_DECOMPOSER
end

# Tests
# Uncommented tests are passing
# Commented tests are failing
#

print "Testing."

a = Record.first

# Original content
assert { a.content == 'ns.example.com root.example.com 2012333335 28800 86400 3600000 86400' }


a.ns      = "newns.example.com"
a.email   = "hostmaster.example.com"
a.serial  = "00000"
a.refresh = "11111"
a.retry   = "22222"
a.expiry  = "33333"
a.minimum = "44444"

assert { a.ns      == 'newns.example.com' }
assert { a.email   == 'hostmaster.example.com' }
assert { a.serial  == '00000'}
assert { a.refresh == '11111' }
assert { a.retry   == '22222' }
assert { a.expiry  == '33333' }
assert { a.minimum == '44444'}

a.save

# Everythings works perfectly below

assert { a.content == 'newns.example.com hostmaster.example.com 00000 11111 22222 33333 44444' }

a.save

a = Record.first

assert { a.content == 'newns.example.com hostmaster.example.com 00000 11111 22222 33333 44444' }

assert { a.ns      == 'newns.example.com' }
assert { a.email   == 'hostmaster.example.com' }
assert { a.serial  == '00000'}
assert { a.refresh == '11111' }
assert { a.retry   == '22222' }
assert { a.expiry  == '33333' }
assert { a.minimum == '44444'}

puts "done"