Question

I know I’ve seen this but I cant find it anywhere.

I have a couple objects with a many-to-many relationship: Person and Department. A person can have many departments and vice versa.

I thought it was:

var person = //query user

var d = new Department();
d.Id = 123;

person.Departments.add(d);

This creates a new department in the database and links them but that isnt what I want. The department already exists. I just want to create the relationship. How can I do this without having to requery the database to get an instance of the department?

Answer 1

Try this:

var department = dbContext.Departments.FirstOrDefault(d => d.Id == 123);
person.Departments.Add(department);
dbContext.SaveChanges();

This shouldn’t query the database until dbContext.SaveChanges() is called as deferred querying is used. However, if that doesn’t work, you can try updating the mapping entity directly instead:

var departmentMember = new DepartmentMember
                           {
                               DepartmentId = 123,
                               MemberId = person.Id
                           };

dbContext.DepartmentMembers.Add(departmentMember);
dbContext.SaveChanges();