module ActiveRecord
  module Locking
    # Active Records support optimistic locking if the field <tt>lock_version</tt> is present.  Each update to the
    # record increments the lock_version column and the locking facilities ensure that records instantiated twice
    # will let the last one saved raise a StaleObjectError if the first was also updated. Example:
    #
    #   p1 = Person.find(1)
    #   p2 = Person.find(1)
    #   
    #   p1.first_name = "Michael"
    #   p1.save
    #   
    #   p2.first_name = "should fail"
    #   p2.save # Raises a ActiveRecord::StaleObjectError
    #
    # You're then responsible for dealing with the conflict by rescuing the exception and either rolling back, merging,
    # or otherwise apply the business logic needed to resolve the conflict.
    #
    # You must ensure that your database schema defaults the lock_version column to 0.
    #
    # This behavior can be turned off by setting <tt>ActiveRecord::Base.lock_optimistically = false</tt>.
    # To override the name of the lock_version column, invoke the <tt>set_locking_column</tt> method.
    # This method uses the same syntax as <tt>set_table_name</tt>
    module Optimistic
      def self.included(base) #:nodoc:
        base.extend ClassMethods

        base.cattr_accessor :lock_optimistically, :instance_writer => false
        base.lock_optimistically = true

        base.alias_method_chain :update, :lock
        base.alias_method_chain :attributes_from_column_definition, :lock

        class << base
          alias_method :locking_column=, :set_locking_column
        end
      end

      def locking_enabled? #:nodoc:
        self.class.locking_enabled?
      end

      private
        def attributes_from_column_definition_with_lock
          result = attributes_from_column_definition_without_lock
        
          # If the locking column has no default value set,
          # start the lock version at zero.  Note we can't use
          # locking_enabled? at this point as @attributes may
          # not have been initialized yet
        
          if lock_optimistically && result.include?(self.class.locking_column)
            result[self.class.locking_column] ||= 0
          end
        
          return result
        end

        def update_with_lock #:nodoc:
          return update_without_lock unless locking_enabled?

          lock_col = self.class.locking_column
          previous_value = send(lock_col)
          send(lock_col + '=', previous_value + 1)

          affected_rows = connection.update(<<-end_sql, "#{self.class.name} Update with optimistic locking")
            UPDATE #{self.class.table_name}
            SET #{quoted_comma_pair_list(connection, attributes_with_quotes(false))}
            WHERE #{self.class.primary_key} = #{quote_value(id)}
            AND #{self.class.quoted_locking_column} = #{quote_value(previous_value)}
          end_sql

          unless affected_rows == 1
            raise ActiveRecord::StaleObjectError, "Attempted to update a stale object"
          end

          return true
        end

      module ClassMethods
        DEFAULT_LOCKING_COLUMN = 'lock_version'

        def self.extended(base)
          class <<base
            alias_method_chain :update_counters, :lock
          end
        end

        # Is optimistic locking enabled for this table? Returns true if the
        # #lock_optimistically flag is set to true (which it is, by default)
        # and the table includes the #locking_column column (defaults to
        # lock_version).
        def locking_enabled?
          lock_optimistically && columns_hash[locking_column]
        end

        # Set the column to use for optimistic locking. Defaults to lock_version.
        def set_locking_column(value = nil, &block)
          define_attr_method :locking_column, value, &block
          value
        end

        # The version column used for optimistic locking. Defaults to lock_version.
        def locking_column
          reset_locking_column
        end

        # Quote the column name used for optimistic locking.
        def quoted_locking_column
          connection.quote_column_name(locking_column)
        end

        # Reset the column used for optimistic locking back to the lock_version default.
        def reset_locking_column
          set_locking_column DEFAULT_LOCKING_COLUMN
        end

        # make sure the lock version column gets updated when counters are
        # updated.
        def update_counters_with_lock(id, counters)
          counters = counters.merge(locking_column => 1) if locking_enabled?
          update_counters_without_lock(id, counters)
        end
      end
    end
  end
end