Model Object / Business Logic question

Question

Hello,
Question about how to architect the model objects and services in our system. We are defining our own Model Object / Transfer Objects without the use of an ORM tool (long story). Some of these model objects have to maintain Association objects such as:
public class Teacher {
     private List<StudentAssociation> kids;
public class StudentAssociation {
     private Teacher teacher;
     private Student student;
     private Date fromDate;
     private Date toDate;
     // Getters / Setters ....
}My question is, where should the logic be to add, remove student associations? Originally we had it defined in the Teacher model object with:
public void addStudentAssociation(Student student, Date fromDate, Date toDate);
public void removeStudentAssociation(Student student, Date fromDate, Date toDate);But I am hesitant to put such logic in the Model Object. I always thought those should be pretty empty of any sort of business logic. Instead I want to have a TeacherService that does that and instead just a getter/setter on the Teacher object for the Association List. However, in doing that, I find I have to create a new List of Associations then call the setAssociations method on the Teacher object, which seems kind of strange.
Is it bad to put the add/remove method in the model object itself? The remove logic has a bit of business logic in it, so it seems weird being there.

Answer 1

Not sure I can be of much help, but here's my two cents worth:
You have a teacher object and student object. A teacher doesnt really have an association with a student. I think you need another object such as a class object. I class has a teacher and the students (an association can be a business association, social association, etc). That same teacher may be assigned to other classes (provided they dont occur at the same time since the teacher cant be in the two classes at the same time. Likewise, for a student. Complicating things is the fact that a teacher of one class may be a student in another class. Also, a class may exist that has students but no assigned teacher yet (your original idea wouldn't be able to handle this since you will need a teacher before you add students). Another case, you have a class without students. Still another case, you have people (either future students, or teaching staff that haven't been assigned as teachers yet), but no classes yet., I think it would be best to figure out a database schema first (you can use Oracle Lite, MySql, etc).
here is an example:
Assuming you are putting this in a database I would create tables and fields something like this:
Person ((a person is either a student or teacher, or just someone that is no longer a student or teacher but may be one day))
personId not null
firstname not null
middleName nullable
lastname not null
ssn not null
Class ( a class is where a teacher teaches students))
classId not null
nameOfCourse not null
building nullable (may not be assigned yet)
room nullable (may not be assigned yet)
startDate ((when the class starts)) nullable
endDate ((when the class ends)) nullable
teacherId ((this is the parentId from the person table)) nullable
Students
personId ((this is the personId from the person table))
classId
associations:
a class has 0 or 1 teacher,
0 or many students
a teacherId must exist in the person table as a personId
a studentId must exist in the person table as a personId
(if you delete a personId in the person table, it cascades deletes any teacherId or studentId of the same value)
your class has a collection of students. therefore:
private List<Student> students
Your class will also have something like
addStudent(Student student) (throw exception if student already exists) (note you dont pass in the start and
end date since its the class's responsibility for start and end date, not student)
isStudent(Student student) (return true if student is already in class, false if not)
deleteStudent(Student student) ((returns true if student found and deleted, false if student not found to delete)
Your business logic can check things such as if the start date occurs before the end date (an error), you have a class with no teacher or no students, etc. The first case you can do in the database with constraints if you want, but the second you cant since you want to store info for a class even if a teacher isn't assigned to it yet.
One way to do this is to have a validate() function in your class object. The validate() function checks such things as a class with no teacher and returns a collection of warnings if it finds anything wrong. All the above is just something off the top of my head. So there may be some issues with my approach that you will have to work out.
Good Luck!