Adding limit/offset to association limiting SQL fragments. Details: http://gist.github.com/25118

Author: metaskills

lib/active_record/connection_adapters/sqlserver_adapter.rb

@@ -305,12 +305,14 @@ def add_limit_offset!(sql, options)
             raise ArgumentError, "limit should be an integer"
           end
         end
-        # The buisiness of adding limit/offset
+        # The business of adding limit/offset
         if options[:limit] and options[:offset]
           total_rows = select_value("SELECT count(*) as TotalRows from (#{sql.gsub(/\bSELECT(\s+DISTINCT)?\b/i, "SELECT#{$1} TOP 1000000000")}) tally").to_i
           if (options[:limit] + options[:offset]) >= total_rows
             options[:limit] = (total_rows - options[:offset] >= 0) ? (total_rows - options[:offset]) : 0
           end
+          # Make sure we do not need a special limit/offset for association limiting. http://gist.github.com/25118
+          add_limit_offset_for_association_limiting!(sql,options) and return if sql_for_association_limiting?(sql)
           # Wrap the SQL query in a bunch of outer SQL queries that emulate proper LIMIT,OFFSET support.
           sql.sub!(/^\s*SELECT(\s+DISTINCT)?/i, "SELECT * FROM (SELECT TOP #{options[:limit]} * FROM (SELECT#{$1} TOP #{options[:limit] + options[:offset]}")
           sql << ") AS tmp1"
@@ -629,6 +631,21 @@ def handle_as_array(handle)
         array
       end
 
+      def add_limit_offset_for_association_limiting!(sql, options)
+        sql.replace %|
+          SET NOCOUNT ON
+          DECLARE @row_number TABLE (row int identity(1,1), id int)
+          INSERT INTO @row_number (id)
+            #{sql}
+          SET NOCOUNT OFF
+          SELECT id FROM (
+            SELECT TOP #{options[:limit]} * FROM (
+              SELECT TOP #{options[:limit] + options[:offset]} * FROM @row_number ORDER BY row
+            ) AS tmp1 ORDER BY row DESC
+          ) AS tmp2 ORDER BY row
+        |.gsub(/[ \t\r\n]+/,' ')
+      end
+      
       # SCHEMA STATEMENTS ========================================#
 
       def remove_check_constraints(table_name, column_name)
@@ -730,6 +747,13 @@ def order_to_min_set(order)
         end.join(', ')
       end
 
+      def sql_for_association_limiting?(sql)
+        if md = sql.match(/^\s*SELECT(.*)FROM.*GROUP BY.*ORDER BY.*/im)
+          select_froms = md[1].split(',')
+          select_froms.size == 1 && !select_froms.first.include?('*')
+        end
+      end
+      
       def remove_sqlserver_columns_cache_for(table_name)
         cache_key = unqualify_table_name(table_name)
         @sqlserver_columns_cache[cache_key] = nil

test/cases/adapter_test_sqlserver.rb

@@ -107,6 +107,27 @@ def setup
       assert !@connection.send(:insert_sql?,'SELECT...')
     end
 
+    context 'for #sql_for_association_limiting?' do
+      
+      should 'return false for simple selects with no GROUP BY and ORDER BY' do
+        assert !sql_for_association_limiting?("SELECT * FROM [posts]")
+      end
+      
+      should 'return true to single SELECT, ideally a table/primarykey, that also has a GROUP BY and ORDER BY' do
+        assert sql_for_association_limiting?("SELECT [posts].id FROM...GROUP BY [posts].id ORDER BY MIN(posts.id)")
+      end
+      
+      should 'return false to single * wildcard SELECT that also has a GROUP BY and ORDER BY' do
+        assert !sql_for_association_limiting?("SELECT * FROM...GROUP BY [posts].id ORDER BY MIN(posts.id)")
+      end
+      
+      should 'return false to multiple columns in the select even when GROUP BY and ORDER BY are present' do
+        sql = "SELECT [accounts].credit_limit, firm_id FROM...GROUP BY firm_id ORDER BY firm_id"
+        assert !sql_for_association_limiting?(sql)
+      end
+      
+    end
+    
     context 'for #get_table_name' do
 
       should 'return quoted table name from basic INSERT, UPDATE and SELECT statements' do
@@ -334,6 +355,10 @@ def setup
 
   private
 
+  def sql_for_association_limiting?(sql)
+    @connection.send :sql_for_association_limiting?, sql
+  end
+  
   def orders_and_dirs_set(order)
     @connection.send :orders_and_dirs_set, order
   end