Fix find_in_batches against string IDs when start option is not specified.

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Fix `find_in_batches` crashing when IDs are strings and start option is not specified.
+
+    *Alexis Bernard*
+
 *   `AR::Base#attributes_before_type_cast` now returns unserialized values for serialized attributes.
 
     *Nikita Afanasenko*

activerecord/lib/active_record/relation/batches.rb

@@ -62,11 +62,11 @@ def find_in_batches(options = {})
         ActiveRecord::Base.logger.warn("Scoped order and limit are ignored, it's forced to be batch order and batch size")
       end
 
-      start = options.delete(:start) || 0
+      start = options.delete(:start)
       batch_size = options.delete(:batch_size) || 1000
 
       relation = relation.reorder(batch_order).limit(batch_size)
-      records = relation.where(table[primary_key].gteq(start)).to_a
+      records = start ? relation.where(table[primary_key].gteq(start)).to_a : relation.to_a
 
       while records.any?
         records_size = records.size

activerecord/test/cases/batches_test.rb

@@ -136,4 +136,13 @@ def test_find_in_batches_should_use_any_column_as_primary_key
 
     assert_equal nick_order_subscribers[1..-1].map(&:id), subscribers.map(&:id)
   end
+
+  def test_find_in_batches_should_use_any_column_as_primary_key_when_start_is_not_specified
+    Subscriber.count('nick') # preheat arel's table cache
+    assert_queries(Subscriber.count + 1) do
+      Subscriber.find_each(:batch_size => 1) do |subscriber|
+        assert_kind_of Subscriber, subscriber
+      end
+    end
+  end
 end