Sequence Property type for DataMapper

sequence.rb

=begin
@author Chris Corbyn
@license None, use at your own risk
=end
module DataMapper
  class Property
    
    # Adds support for a sequences table to DataMapper, via a Sequence property type.
    # 
    # class Person
    #   include DataMapper::Resource
    #   
    #   property :id,   Sequence
    # end
    # 
    # You can optionally specify a sequence name on the propery, if it doesn't match convention.
    # 
    # class Person
    #   include DataMapper::Resource
    #   
    #   property :id,   Sequence, :sequence_name => 'person_personid_seq'
    # end
    #
    # The standard naming convention is "#{table_name}_#{field_name}_seq".
    class Sequence < Integer
      Property.accept_options(:sequence_name)
      
      key     true
      min     1
      default lambda { |resource, property| property.next_sequence }
      
      @@sequences_table = 'sequences'
      @@sequences_name  = 'name'
      @@sequences_id    = 'id'
      
      @sequence_name = nil
      
      @retrying = false
      
      def self.table(table)
        @@sequences_table = table.to_s
      end
      
      def self.name_column(name)
        @@sequences_name = name.to_s
      end
      
      def self.id_column(name)
        @@sequences_id = name.to_s
      end
      
      def initialize(model, name, options = {})
        field_name = options.fetch(:field, name).to_s
        @sequence_name = options.fetch(:sequence_name, sequence_name_for(model.storage_name, field_name))
        
        super
      end
        
      def next_sequence
        adapter = repository.adapter
        sql = "SELECT #{@@sequences_id} FROM #{@@sequences_table} WHERE #{@@sequences_name} = ? FOR UPDATE"
        
        sequence = adapter.select(sql, @sequence_name).first
        if sequence
          sql = "UPDATE #{@@sequences_table} SET #{@@sequences_id} = #{@@sequences_id} + 1 WHERE #{@@sequences_name} = ?"
        else
          sql = "INSERT INTO #{@@sequences_table} (#{@@sequences_name}, #{@@sequences_id}) VALUES (?, 1)"
          sequence = 1
        end
        
        adapter.execute(sql, @sequence_name)
        
        @retrying = false
        
        sequence
      rescue ::DataObjects::SQLError
        unless @retrying
          @retrying = true
          retry
        else
          @retrying = false
          raise
        end
      end
      
      private
        
        def sequence_name_for(table, column)
          "#{table}_#{column}_seq"
        end
    end
    
  end
end