Pulled in ActiveRecord synchronize functionality from the old ar-extensions code base.

https://github.com/zdennis/activerecord-import/issues#issue/6

lib/activerecord-import/base.rb

@@ -25,3 +25,4 @@ end
 this_dir = Pathname.new File.dirname(__FILE__)
 require this_dir.join("import")
 require this_dir.join("active_record/adapters/abstract_adapter")
+require this_dir.join("synchronize")

lib/activerecord-import/synchronize.rb

@@ -0,0 +1,30 @@
+module ActiveRecord # :nodoc:
+  class Base # :nodoc:
+      
+    # Synchronizes the passed in ActiveRecord instances with data
+    # from the database. This is like calling reload
+    # on an individual ActiveRecord instance but it is intended for use on
+    # multiple instances. 
+    #
+    # This uses one query for all instance updates and then updates existing
+    # instances rather sending one query for each instance
+    def self.synchronize(instances, key=self.primary_key)
+      return if instances.empty?
+      
+      keys = instances.map(&"#{key}".to_sym)
+      klass = instances.first.class
+      fresh_instances = klass.find( :all, :conditions=>{ key=>keys }, :order=>"#{key} ASC" )
+
+      instances.each_with_index do |instance, index|
+        instance.clear_aggregation_cache
+        instance.clear_association_cache
+        instance.instance_variable_set '@attributes', fresh_instances[index].attributes
+      end
+    end
+
+    # See ActiveRecord::ConnectionAdapters::AbstractAdapter.synchronize
+    def synchronize(instances, key=ActiveRecord::Base.primary_key)
+      self.class.synchronize(instances, key)
+    end
+  end
+end

test/synchronize_test.rb

@@ -0,0 +1,22 @@
+require File.expand_path('../test_helper', __FILE__)
+
+describe ".synchronize" do
+  let(:topics){ Generate(3, :topics) }
+  let(:titles){ %w(one two three) }
+  
+  setup do
+    # update records outside of ActiveRecord knowing about it
+    Topic.connection.execute( "UPDATE #{Topic.table_name} SET title='#{titles[0]}_haha' WHERE id=#{topics[0].id}", "Updating record 1 without ActiveRecord" )
+    Topic.connection.execute( "UPDATE #{Topic.table_name} SET title='#{titles[1]}_haha' WHERE id=#{topics[1].id}", "Updating record 2 without ActiveRecord" )
+    Topic.connection.execute( "UPDATE #{Topic.table_name} SET title='#{titles[2]}_haha' WHERE id=#{topics[2].id}", "Updating record 3 without ActiveRecord" )
+  end
+  
+  it "reloads data for the specified records" do
+    Book.synchronize topics
+
+    actual_titles = topics.map(&:title)
+    assert_equal "#{titles[0]}_haha", actual_titles[0], "the first record was not correctly updated"
+    assert_equal "#{titles[1]}_haha", actual_titles[1], "the second record was not correctly updated"
+    assert_equal "#{titles[2]}_haha", actual_titles[2], "the third record was not correctly updated"
+  end
+end