Merge remote branch 'rsim/habtm_select_fix'

* rsim/habtm_select_fix:
  Explicitly select * from has_and_belongs_to_many association tables, simplify exists? query

activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb

@@ -83,7 +83,8 @@ module ActiveRecord
           super.merge(
             :joins    => construct_joins,
             :readonly => ambiguous_select?(@reflection.options[:select]),
-            :select   => @reflection.options[:select] || Arel.star
+            :select   => @reflection.options[:select] ||
+              Arel.sql("#{@reflection.quoted_table_name}.*, #{@owner.connection.quote_table_name @reflection.options[:join_table]}.*")
           )
         end
 

activerecord/lib/active_record/relation/finder_methods.rb

@@ -171,7 +171,7 @@ module ActiveRecord
     def exists?(id = nil)
       id = id.id if ActiveRecord::Base === id
 
-      relation = select(table[primary_key]).limit(1)
+      relation = select("1").limit(1)
 
       case id
       when Array, Hash