PostgreSQL: use a subselect to correctly perform eager finds with :limit and :order. Closes #4668.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@5887 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

1 files changed, 16 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index c0efd1a341..5ef117a749 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -383,17 +383,29 @@ module ActiveRecord
 
         # construct a clean list of column names from the ORDER BY clause, removing
         # any asc/desc modifiers
-        order_columns = order_by.split(',').collect! { |s| s.split.first }
+        order_columns = order_by.split(',').collect { |s| s.split.first }
         order_columns.delete_if &:blank?
-
-        # add the DISTINCT columns to the start of the ORDER BY clause
-        order_by.replace "#{columns}, #{order_by}"
+        order_columns = order_columns.zip((0...order_columns.size).to_a).map { |s,i| "#{s} AS alias_#{i}" }
 
         # return a DISTINCT ON() clause that's distinct on the columns we want but includes
         # all the required columns for the ORDER BY to work properly
         sql = "DISTINCT ON (#{columns}) #{columns}, "
         sql << order_columns * ', '
       end
+      
+      # ORDER BY clause for the passed order option.
+      # 
+      # PostgreSQL does not allow arbitrary ordering when using DISTINCT ON, so we work around this
+      # by wrapping the sql as a sub-select and ordering in that query.
+      def add_order_by_for_association_limiting!(sql, options)
+        return sql if options[:order].blank?
+        
+        order = options[:order].split(',').collect { |s| s.strip }.reject(&:blank?)
+        order.map! { |s| 'DESC' if s =~ /\bdesc$/i }
+        order = order.zip((0...order.size).to_a).map { |s,i| "id_list.alias_#{i} #{s}" }.join(', ')
+        
+        sql.replace "SELECT * FROM (#{sql}) AS id_list ORDER BY #{order}"
+      end
 
       private
         BYTEA_COLUMN_TYPE_OID = 17