db={services:[{"_id":ObjectId("61dad1d21aa077c61b7bc2aa"),"name":"HomeMaintenance","subServices":[ObjectId("61dacb86cb94917c1edcea8f"),ObjectId("61dad5812881410ba441c401")],},{"_id":ObjectId("61dad60b2881410ba441c40e"),"name":"HomeMaintenance","subServices":[],}],sub_services:[{"_id":ObjectId("61dacb86cb94917c1edcea8f"),"name":"something","title":"something else","imageUrl":"","__v":0,"service":"61dad1d21aa077c61b7bc2aa"},{"_id":ObjectId("61dad5812881410ba441c401"),"name":"Plumbing","title":"Plumbing","imageUrl":"","__v":0,"service":"61dad1d21aa077c61b7bc2aa"}]}

db.services.aggregate([{$match:{_id:ObjectId("61dad1d21aa077c61b7bc2aa")},},{$lookup:{from:"sub_services",let:{subServices:"$subServices"},pipeline:[{$match:{$expr:{$in:["$_id","$$subServices"]},},},],as:"subServices",},},])