db={"categories":[{id:65,title:"Automotive",active:true,subcategories:[35,28,30]}],"subcategories":[{id:35,title:"Automotive technology",category_id:65,active:true},{id:28,title:"Automotive coatings",category_id:65,active:true},{id:30,title:"Machinery and equipments",category_id:65,active:true},{id:31,title:"Machinery and equipments",category_id:66,active:true}]}

db.categories.aggregate([{"$lookup":{"from":"subcategories","localField":"subcategories","foreignField":"id","as":"subcategories"}}])