1 files changed, 23 insertions, 12 deletions
diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index c1842b1a96..869eebfa34 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -196,24 +196,22 @@ module ActiveRecord
     end
 
     def execute_simple_calculation(operation, column_name, distinct) #:nodoc:
-      column = aggregate_column(column_name)
-
       # Postgresql doesn't like ORDER BY when there are no GROUP BY
       relation = except(:order)
-      select_value = operation_over_aggregate_column(column, operation, distinct)
 
-      relation.select_values = [select_value]
+      if operation == "count" && (relation.limit_value || relation.offset_value)
+        # Shortcut when limit is zero.
+        return 0 if relation.limit_value == 0
 
-      query_builder = relation.arel
+        query_builder = build_count_subquery(relation, column_name, distinct)
+      else
+        column = aggregate_column(column_name)
 
-      if operation == "count"
-        limit  = relation.limit_value
-        offset = relation.offset_value
+        select_value = operation_over_aggregate_column(column, operation, distinct)
 
-        unless limit && offset
-          query_builder.limit  = nil
-          query_builder.offset = nil
-        end
+        relation.select_values = [select_value]
+
+        query_builder = relation.arel
       end
 
       type_cast_calculated_value(@klass.connection.select_value(query_builder.to_sql), column_for(column_name), operation)
@@ -312,5 +310,18 @@ module ActiveRecord
         select if select !~ /(,|\*)/
       end
     end
+
+    def build_count_subquery(relation, column_name, distinct)
+      column_alias = Arel.sql('count_column')
+      subquery_alias = Arel.sql('subquery_for_count')
+
+      aliased_column = aggregate_column(column_name == :all ? 1 : column_name).as(column_alias)
+      relation.select_values = [aliased_column]
+      subquery = relation.arel.as(subquery_alias)
+
+      sm = Arel::SelectManager.new relation.engine
+      select_value = operation_over_aggregate_column(column_alias, 'count', distinct)
+      sm.project(select_value).from(subquery)
+    end
   end
 end