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":{"username":{"$regex":"^leo","$options":"i"}}}]}}])