Database
db={"emp":[{"_id":{"$oid":"610bcc467b0c4008346547b8"},"name":"xyz","email":"xyz@gmail.com","password":"xyz","gender":"MALE","deptID":{"$oid":"610bce417b0c4008346547bc"},}],"dept":[{"_id":{"$oid":"610bce417b0c4008346547bc"},"name":"Chicago","number":10}]}
Query
db.dept.aggregate([{"$lookup":{"from":"emp","localField":"_id","foreignField":"deptID","as":"employees"}}])