An attempt with Sequel's composition plugin

record.rb

#
# The purpose of the database is to hold nameserver zone records
# 
# Unfortunately, the SOA type record contains several interesting space-separated
# "inner" fields we would like to access individually.
#
# The point is is to "autovivify" getters and
# setters for these "inner fields"
#
# I'll try to solve this using Sequel's composition plugin
# described in the Sequel documentation and announcement post :
# - http://sequel.heroku.com/2010/03/08/composition-plugin-added/
# - http://sequel.rubyforge.org/rdoc-plugins/classes/Sequel/Plugins/Composition.html
#
# Unfortunately, the example in the documentation does the opposite :
# it builds one getter/setter from multiple record columns from on field,
# while we want to build multiple getter/setters from on record column
#
# This is my unsuccessful attempt to make it work, thanks for reading & feel free to comment
#
########

# 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
  @@soalist = [ :ns, :email, :serial, :refresh,
    :retry, :expiry, :minimum ]

  # We will need on composer per field, lets create a Hash to store the 
  # composer procs inside
  @@soa_composer = {}

  # Composers return the values we want from the underlying record
  # Creates a composer proc for each field
  @@soalist.each do |k|
    @@soa_composer[k] = proc do
      # We only do this for SOA type records
      self.content.split()[@@soalist.index(k)] if self.type == 'SOA'
    end
  end

  # Decomposer builds the record column at save time
  # We'll just need one decomposer, identical for all composers
  soa_decomposer = proc do
    @@soalist.each do |f|
      #puts "deco #{f}, value is #{compositions[f]} or #{self.send(f)}"
      #puts "\t<=#{self.content}"

      # We split self.content, so we can replace the required inner field
      # Thanks to @@soalist, we can map inner values easily 
      # There's probably a more elegant way though
      fields = self.content.split
      fields[@@soalist.index(f)] = self.send(f)
      self.content = fields.join(' ')             
      #puts "\t=>#{self.content}"
    end
  end

  # Create '@@soalist.count' composers, one for each 'iner field'
  @@soalist.each do |k|
    composition k,
      :composer => @@soa_composer[k],
      :decomposer => soa_decomposer
  end
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"

# 1st downside : asserting before save will fail
#
#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'}

# 2nd problem : when save is called, the decomposer will
# be called for each inner item. This is quite suboptimal
# since the decomposer takes care of each field everytime
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"

__END__

# May be it would be easier if we could pass a parameter to the composer,
# something, along the lines, that would look like this :
#
#
  # Composer
  soa_composer = proc do |f|
    # We only do this for SOA type records
    self.content.split()[@@soalist.index(f)] if self.type == 'SOA'
  end
  # Decomposer builds the record column at save time
  # We'll just need one decomposer, identical for all composers
  soa_decomposer = proc do |f|
    fields = self.content.split
    fields[@@soalist.index(f)] = self.send(f)
    self.content = fields.join(' ') 
  end

  # Create a composer/decomposer for each inner field
  # The {de,}composer receives the field symbol as the first argument
  # and acts accordingly
  @@soalist.each do |k|
    composition k,
      :composer => soa_composer(k),
      :decomposer => soa_decomposer(k)
  end