db={users:[{_id:1,username:"mark"},{_id:2,username:"leon"},{_id:3,username:"leontin"}],data:[{_id:ObjectId("6675309f82bdfbe3b00febfa"),user:1,message:"94451 Laos Sweden Iran"},{_id:ObjectId("6675309f82bdfbe3b00febfb"),user:2,message:"message 22222"},{_id:ObjectId("6675309f82bdfbe3b00febfc"),user:3,message:"message 3333"}]}

db.data.aggregate([{$lookup:{from:"users",localField:"user",foreignField:"_id",as:"user_info",pipeline:[{$match:{$expr:{$regexMatch:{input:"$username",regex:"^leo",options:"i"}}}}]}},{$unwind:"$user_info"}])