1 files changed, 143 insertions, 0 deletions
diff --git a/activerecord/lib/arel/visitors/mssql.rb b/activerecord/lib/arel/visitors/mssql.rb
new file mode 100644
index 0000000000..fdd864b40d
--- /dev/null
+++ b/activerecord/lib/arel/visitors/mssql.rb
@@ -0,0 +1,143 @@
+# frozen_string_literal: true
+
+module Arel # :nodoc: all
+  module Visitors
+    class MSSQL < Arel::Visitors::ToSql
+      RowNumber = Struct.new :children
+
+      def initialize(*)
+        @primary_keys = {}
+        super
+      end
+
+      private
+
+        def visit_Arel_Nodes_IsNotDistinctFrom(o, collector)
+          right = o.right
+
+          if right.nil?
+            collector = visit o.left, collector
+            collector << " IS NULL"
+          else
+            collector << "EXISTS (VALUES ("
+            collector = visit o.left, collector
+            collector << ") INTERSECT VALUES ("
+            collector = visit right, collector
+            collector << "))"
+          end
+        end
+
+        def visit_Arel_Nodes_IsDistinctFrom(o, collector)
+          if o.right.nil?
+            collector = visit o.left, collector
+            collector << " IS NOT NULL"
+          else
+            collector << "NOT "
+            visit_Arel_Nodes_IsNotDistinctFrom o, collector
+          end
+        end
+
+        def visit_Arel_Visitors_MSSQL_RowNumber(o, collector)
+          collector << "ROW_NUMBER() OVER (ORDER BY "
+          inject_join(o.children, collector, ", ") << ") as _row_num"
+        end
+
+        def visit_Arel_Nodes_SelectStatement(o, collector)
+          if !o.limit && !o.offset
+            return super
+          end
+
+          is_select_count = false
+          o.cores.each { |x|
+            core_order_by = row_num_literal determine_order_by(o.orders, x)
+            if select_count? x
+              x.projections = [core_order_by]
+              is_select_count = true
+            else
+              x.projections << core_order_by
+            end
+          }
+
+          if is_select_count
+            # fixme count distinct wouldn't work with limit or offset
+            collector << "SELECT COUNT(1) as count_id FROM ("
+          end
+
+          collector << "SELECT _t.* FROM ("
+          collector = o.cores.inject(collector) { |c, x|
+            visit_Arel_Nodes_SelectCore x, c
+          }
+          collector << ") as _t WHERE #{get_offset_limit_clause(o)}"
+
+          if is_select_count
+            collector << ") AS subquery"
+          else
+            collector
+          end
+        end
+
+        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 visit_Arel_Nodes_DeleteStatement(o, collector)
+          collector << "DELETE "
+          if o.limit
+            collector << "TOP ("
+            visit o.limit.expr, collector
+            collector << ") "
+          end
+          collector << "FROM "
+          collector = visit o.relation, collector
+          if o.wheres.any?
+            collector << " WHERE "
+            inject_join o.wheres, collector, AND
+          else
+            collector
+          end
+        end
+
+        def determine_order_by(orders, x)
+          if orders.any?
+            orders
+          elsif x.groups.any?
+            x.groups
+          else
+            pk = find_left_table_pk(x.froms)
+            pk ? [pk] : []
+          end
+        end
+
+        def row_num_literal(order_by)
+          RowNumber.new order_by
+        end
+
+        def select_count?(x)
+          x.projections.length == 1 && Arel::Nodes::Count === x.projections.first
+        end
+
+        # FIXME raise exception of there is no pk?
+        def find_left_table_pk(o)
+          if o.kind_of?(Arel::Nodes::Join)
+            find_left_table_pk(o.left)
+          elsif o.instance_of?(Arel::Table)
+            find_primary_key(o)
+          end
+        end
+
+        def find_primary_key(o)
+          @primary_keys[o.name] ||= begin
+            primary_key_name = @connection.primary_key(o.name)
+            # some tables might be without primary key
+            primary_key_name && o[primary_key_name]
+          end
+        end
+    end
+  end
+end