Add support for counting rows with conditions or limit using a subquery.

lib/arel/engines/sql/relations/compiler.rb

@@ -8,17 +8,28 @@ def initialize(relation)
       end
 
       def select_sql
-        query = build_query \
-          "SELECT     #{select_clauses.join(', ')}",
-          "FROM       #{from_clauses}",
+        if relation.projections.first.is_a?(Count) && relation.projections.size == 1 && (taken.present? || wheres.present?)
+          subquery = build_query("SELECT 1 FROM #{from_clauses}", build_clauses)
+          query = "SELECT #{select_clauses.join(', ')} FROM (#{subquery})"
+        else
+          query = build_query \
+            "SELECT     #{select_clauses.join(', ')}",
+            "FROM       #{from_clauses}",
+            build_clauses
+        end
+        query
+      end
+
+      def build_clauses
+        clauses = build_query "",
           (joins(self)                                   unless joins(self).blank? ),
           ("WHERE     #{where_clauses.join(' AND ')}"    unless wheres.blank?      ),
           ("GROUP BY  #{group_clauses.join(', ')}"       unless groupings.blank?   ),
           ("HAVING    #{having_clauses.join(' AND ')}"      unless havings.blank?     ),
           ("ORDER BY  #{order_clauses.join(', ')}"       unless orders.blank?      )
-          engine.add_limit_offset!(query,{ :limit => taken, :offset => skipped }) if taken || skipped
-          query << " #{locked}" unless locked.blank?
-          query
+        clauses << " #{locked}" unless locked.blank?
+        engine.add_limit_offset!(clauses,{ :limit => taken, :offset => skipped }) if taken || skipped
+        clauses unless clauses.blank?
       end
 
       def delete_sql

spec/engines/sql/unit/relations/take_spec.rb

@@ -52,6 +52,24 @@ module Arel
           })
         end
       end
+
+      it "manufactures count sql with limit" do
+        sql = Take.new(@relation.project(@relation[:id].count), @taken).to_sql
+
+        adapter_is :mysql do
+          sql.should be_like(%Q{
+            SELECT COUNT(`users`.`id`) AS count_id
+            FROM (SELECT 1 FROM `users` LIMIT 4)
+          })
+        end
+
+        adapter_is_not :mysql, :oracle do
+          sql.should be_like(%Q{
+            SELECT COUNT("users"."id") AS count_id
+            FROM (SELECT 1 FROM "users" LIMIT 4)
+          })
+        end
+      end
     end
   end
 end