Many-to-one relationship in MongoDB

Question

I am new to MongoDB and NoSQL databases and I am trying to learn different kind of thinking that everybody mentions when it comes to NoSQL.

I have a typical situation with many-to-one relationship. Please don't tell me that MongoDB is not a relational database, because I know that already. Key point is - reality works certain way and I need to make my application to reflect it. Real world is full of relationships and if your answer is "choose a different DB for your case" then I think that MongoDB team could close their business, because their product would be completely useless in such a case.

So let's assume typical Employee / Department relation. Employee is linked to a single Department . Department can have zero or multiple Employees .

Let's assume super simple models:

public class Department {

    private String name;

   // something like...
   private List<Employee> employees;

}

public class Employee {

    private String name;

   // something like...
   private Department department;

}

Now in my REST API I need few quite basic functionalities:

• get list of all departments
• get list of all employees, but it must contain name of department for each employee
• get list of employees for a selected department

So how would you solve this with MongoDB? I am also using Spring Boot with Spring Data, but I think it doesn't matter very much.

I have come across different approaches, but all seem very bad to me. Embedding department inside employee will make it impossible to get list of all departments or employees of a single department. Embedding employees in department will make it impossible to get list of all employees. Using @DbRef will simulate the relation, but then how do I get list of employees including the department names without calling REST API for each employee?

I have read quite a lot of tutorials, manuals and StackOverflow discussions recently, but I haven't found acceptable answer. Is it really possible that MongoDB can't solve such a super-simple problem? It is not even a problem , it is rather a standard - common situation in the world around us.

thank you

Answer 1

Maybe you can use a simple departmentId (with the value of the Mongo ID for the department) in your Employee document?

This way you avoid the embedded document and your use cases can work :

1. All departments case is simply a find-all in the departments collection.
2. All employees is a find-all in the employees collection and then in your server you need to call the department collection for all the distinct department ids that exist in the employees.
3. All employees for a selected department case is a find-by-departmentId in the employees collection.

Would this work for you in an acceptable manner?