This is a design for employees to ask for leave.
There is a need to obtain leave data for all employees.
There will also be a need to obtain leave data for a single employee.
Which is better? Should we just leave the leaveId and create a new collection?
The second good point is that nosql can effectively improve the efficiency by flattening the data and even reducing the correlation with a proper amount of redundancy. This is the so-called anti-normal form design. There are many types of relational databases, the so-called normal form design.