drawohara - Rails: ActiveRecord default values

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drawohara @twatter
Rails: ActiveRecord default values

ActiveRecord is an amazing peice of software but one thing it lacks is an easy
mechanism for setting column defaults outside of the database and virtually no
mechanism for setting defaults via database functions.  Basically there is no
easy way to do this

   class Model < ActiveRecord::Base
     defaults ‘time’ => ‘now()’
   end

The first thing we need is a generic mechanism for setting defaults.  This
does nicely:

   class ActiveRecord::Base
     alias_method ‘__initialize__’, ‘initialize’

    def initialize options = nil, &block
       returning( __initialize__(options, &block) ) do
         options ||= {}
         options.to_options!
         defaults = self.class.defaults || self.defaults || Hash.new 
         (defaults.keys - options.keys).each do |key|
           value = defaults[key]
           case value
             when Proc
               value = instance_eval &value
             when Symbol
               value = send value 
           end
           send “#{ key }=”, value 
         end
       end
     end

    def self.defaults *argv
       @defaults = argv.shift.to_hash if argv.first 
       return @defaults if defined? @defaults
     end

    def defaults *argv
       @defaults = argv.shift.to_hash if argv.first 
       return @defaults if defined? @defaults
     end
   end

Now we can do this

   class Model < ActiveRecord::Base
     defaults ‘foo’ => 42
   end

and this

   class Model < ActiveRecord::Base
     defaults ‘foo’ => lambda{ 42 } 
   end

and this

   class Model < ActiveRecord::Base
     defaults ‘foo’ => :bar 
    def bar() 42 end
   end

and this

   class Model < ActiveRecord::Base
     defaults ‘foo’ => lambda{ bar }
    def bar() 42 end
   end

now, sprinkling in a little abuse of the rails’ source we do

   class ActiveRecord::Base
    module UnQuoted
      def quoted_id() self end ### hackity hack, don’t talk back
     end

     DEFAULT = ‘DEFAULT’
     DEFAULT.extend UnQuoted
     DEFAULT.freeze

    def pgsql(*argv, &block)
       string = argv.join(’ ’)
       string += “ #{ block.call }” if block
       string.extend UnQuoted
       string
     end

    def self.nextval seq
       connection = ActiveRecord::Base.connection
       lambda{ 
         Integer(connection.execute(”select nextval(‘#{ seq }’)”)[0][0])
       }
     end

    def nextval *a, &b 
       self.class.nextval *a, &b
     end
   end

and we’re all good to do things like

   class Model < ActiveRecord::Base
   ### use the db default, even function
     defaults ‘time’ => DEFAULT
   end
 

and

   class Model < ActiveRecord::Base
   ### use arbitrary sql for default value
     defaults ‘time’ => pgsql(’now()’) 
   end




and

   class Model < ActiveRecord::Base
   ### yank a value out of a sequence as the default 
     defaults ‘voter_id’ => nextval(’voter_id_seq’) 
   end

All in all a lot of bang for not much code.  Enjoy.
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