如何建立收藏夹文件之间的关系

Question

 this is the structure of my collections sales collection {
     "_id" : ObjectId("5bfb6ee3636239094b2fb6c3"),
    "items" : [ 
         {
             "_id" : ObjectId("5bfbaa776362390f8f28bf42"),
            "item_id" : 15703,
             "name" : "06455kr3404",
             "code" : "18201",
             "quantity" : 1,
             "price_per_unit" : 100000,
             "total_bef_disc" : 100000,
             "total_aft_disc" : 88000,
             "disabled" : false,
             "inventorable" : false,
             "average_cost" : 0,
             "disc" : 12000,
            "location" : null,
             "updated_at" : ISODate("2018-12-11T06:48:07.000Z"),
             "cogs_per_unit" : 0,
             "total_cogs" : 0
         }
     ],
     "total_bef_disc" : 123000,
     "total_aft_disc" : 108460,
     "tota_payment" : 0,
     "disc" : 14540,
     "difference" : 81540,
     "updated_at" : ISODate("2018-12-11T06:48:07.000Z"),
     "created_at" : ISODate("2018-11-26T03:56:19.000Z"),
     "total_payment" : 190000,
     "total_cogs" : 5882,

 "closed_at" : ISODate("2018-12-11T06:48:07.000Z") } 

  
  this collection sales_retur` {
       "_id" : ObjectId("5c1c7e4363623908f66f9214"),
       "sales_id" : "5bfb6ee3636239094b2fb6c3",
       "total_return" : 111111,
       "updated_at" : ISODate("2018-12-21T05:56:19.000Z"),
       "created_at" : ISODate("2018-12-21T05:46:43.000Z"),
       "reason" : null,
       "inventory_confirmation_required" : false,
       "total_payment" : 0,
       "items" : [ 
           {
               "_id" : ObjectId("5bfbaa776362390f8f28bf42"),
               "item_id" : 4,
               "name" : "06455kr3404",
               "code" : "13538KGH900",
               "quantity" : 1,
               "total_return" : 111111,
               "return_price_per_unit" : 111111,
               "max_total_return" : 17600,
               "updated_at" : ISODate("2018-12-21T05:56:19.000Z")
           }
       ] }   
  how to make a relationship between embeds items in a collection sales  with embeds items in collateral sales returns ?