LIMIT and OFFSET support for MS SQL

1 files changed, 69 insertions, 9 deletions

diff --git a/lib/arel/visitors/mssql.rb b/lib/arel/visitors/mssql.rb
index ea7ab6394c..713ad0f0f8 100644
--- a/lib/arel/visitors/mssql.rb
+++ b/lib/arel/visitors/mssql.rb
@@ -3,19 +3,79 @@ module Arel
     class MSSQL < Arel::Visitors::ToSql
       private
 
-      def build_subselect key, o
-        stmt     = super
-        core     = stmt.cores.first
-        core.top = Nodes::Top.new(o.limit.expr) if o.limit
-        stmt
+      # `top` wouldn't really work here. I.e. User.select("distinct first_name").limit(10) would generate
+      # "select top 10 distinct first_name from users", which is invalid query! it should be
+      # "select distinct top 10 first_name from users"
+      def visit_Arel_Nodes_Top o
+        ""
       end
 
-      def visit_Arel_Nodes_Limit o
-        ""
+      def visit_Arel_Nodes_SelectStatement o
+        if !o.limit && !o.offset
+          return super o
+        end
+
+        select_order_by = "ORDER BY #{o.orders.map { |x| visit x }.join(', ')}" unless o.orders.empty?
+
+        is_select_count = false
+        sql = o.cores.map { |x|
+          core_order_by = select_order_by || determine_order_by(x)
+          if select_count? x
+            x.projections = [row_num_literal(core_order_by)]
+            is_select_count = true
+          else
+            guard_against_select_constant! x
+            x.projections << row_num_literal(core_order_by)
+          end
+
+          visit_Arel_Nodes_SelectCore x
+        }.join
+
+        sql = "SELECT _t.* FROM (#{sql}) as _t WHERE #{get_offset_limit_clause(o)}"
+        # fixme count distinct wouldn't work with limit or offset
+        sql = "SELECT COUNT(1) as count_id FROM (#{sql}) AS subquery" if is_select_count
+        sql
       end
 
-      def visit_Arel_Nodes_Top o
-        "TOP #{visit o.expr}"
+      def get_offset_limit_clause o
+        first_row = o.offset ? o.offset.expr.to_i + 1 : 1
+        last_row  = o.limit ? o.limit.expr.to_i - 1 + first_row : nil
+        if last_row
+          " _row_num BETWEEN #{first_row} AND #{last_row}"
+        else
+          " _row_num >= #{first_row}"
+        end
+      end
+
+      def determine_order_by x
+        unless x.groups.empty?
+          "ORDER BY #{x.groups.map { |g| visit g }.join ', ' }"
+        else
+          "ORDER BY #{find_left_table_pk(x.froms)}"
+        end
+      end
+
+      def row_num_literal order_by
+        Nodes::SqlLiteral.new("ROW_NUMBER() OVER (#{order_by}) as _row_num")
+      end
+
+      def select_count? x
+        x.projections.length == 1 && Arel::Nodes::Count === x.projections.first
+      end
+
+      def guard_against_select_constant! x
+        # guard against .select(1) (i.e. validate_uniqueness uses it to minimize qry result set)
+        # todo it won't work for .select('a'), which is probably ok. 'coz of workaround: .select("'a' as a")
+        x.projections.map! do |p|
+          p.kind_of?(Fixnum) ? Nodes::SqlLiteral.new("#{p} as _fld_#{p}") : p
+        end
+      end
+
+      # fixme raise exception of there is no pk?
+      # fixme!! Table.primary_key will be depricated. What is the replacement??
+      def find_left_table_pk o
+        return visit o.primary_key if o.instance_of? Arel::Table
+        find_left_table_pk o.left if o.kind_of? Arel::Nodes::Join
       end
 
     end