db={"Collection1":[{userID:"1",container:[{"item":false,"price":NumberDecimal("80"),"sizes":"S","arrayIndex":NumberLong(0)},{"item":false,"price":NumberDecimal("80"),"sizes":"M","arrayIndex":NumberLong(1)},{"item":true,"price":NumberDecimal("80"),"sizes":"L","arrayIndex":NumberLong(2)}]}],"main":[{result:"1"}]}

db.main.aggregate([{$lookup:{from:"Collection1",localField:"result",foreignField:"userID",as:"photos"}},{$unwind:{path:"$photos",preserveNullAndEmptyArrays:true}},{$unwind:{path:"$photos.container",preserveNullAndEmptyArrays:true}},{$project:{"photos":{$cond:[{$eq:["$photos.container.item",false]},"$photos.container.price",null]}}},])