MTM relationship in EF CodeFirst - updating woes - entity-framework

Question

I am having a problem with an Entity Framework 6.1.3 CodeFirst Many-to-Many relationship. My model is essentially like so:
class Schedule
{
int Id { get; set; }
}
class Contest
{
int Id { get; set; }
ICollection<Schedule> GameSchedules { get; set; }
}
My context for reference:
class MyContext
{
MyContext() : base("name=DefaultConnection")
{
// no lazy loading for us
this.Configuration.LazyLoadingEnabled = false;
// do not auto detect changes for me
this.Configuration.AutoDetectChangesEnabled = false;
// we don't want our stuff to be wrapped in proxies
this.Configuration.ProxyCreationEnabled = false;
}
}
Entity Farmework CodeFirst configuraiton:
class ContestConfiguration : EntityTypeConfiguration<Contest>
{
ContestConfiguration()
{
// setup many-to-many table between game schedules and contests
this.HasMany(contest => contest.GameSchedules)
.WithMany(schedule => schedule.Contests)
.Map(
contestSchedule =>
{
contestSchedule.MapLeftKey("ContestId");
contestSchedule.MapRightKey("ScheduleId");
contestSchedule.ToTable("ContestSchedule", "something");
});
}
}
Upon creating a contest with an existing schedule I do the following and I see taht two sql statements went in, one that creates the contest and one that creates a record for the MTM table.
Contest Add(Contest entity)
{
// setup schedules
entity.GameSchedules.ToList().ForEach(schedule => this.Context.Entry(schedule).State = EntityState.Unchanged);
// call base add method
return base.Add(entity);
}
However, when I try to Update, its a very different story. I have tried numerous ways, and cannot get CodeFirst to update the relationshiop in the MTM table. It either tries to delete a schedule along with the MTM record or it does nothing. Any ideas on how to accomplish this mind blowing feat?

Answer 1

Ok, figured it out. First and foremost, remove the ManyToManyCascadeDeleteConvention or else on each DELETE from the MTM table, it will try to remove the schedule or contest as well. This is because by default EntityFramework is setting the cascade delete to true.
// remove many-to-many cascade auto delete
modelBuilder.Conventions.Remove<ManyToManyCascadeDeleteConvention>();
now here is the code that updates
1. Ensure that you have copied over the new schedules to a new collection
2. Modify the existing entity with new data (this can be done either before or after the MTM table is updated)
3. To make it simple, I just removed all of the current rows from the MTM table that are linked to this and added the new ones for the sake of brevity.
4. Save your changes to make sure the schedules are deleted
5. Now add the new ones
6. Save your changes again to make sure the new schedules are added
public void Update(Contest entity)
{
// (1) generate new collection so that the reference doesn't get overwritten
var schedulesToAdd = entity.GameSchedules.ToList();
// (2) get entry
var entry = this.context.Entry(entity);
// set entry to modified
entry.State = EntityState.Modified;
// save the current state
this.Context.SaveChanges();
// get the existing context from the db
var existingContest = this.Set.Where(contest => contest.Id == entity.Id).Include(contest => contest.GameSchedules).SingleOrDefault();
// get the object context
var context = ((IObjectContextAdapter)this.Context).ObjectContext;
// iterate over existing relationships and eliminate
foreach (var existingSchedule in existingContest.GameSchedules.ToList())
{
var schedule = this.Context.Schedules.Find(existingSchedule.Id);
// (3) mark relationship as deleted
context.ObjectStateManager.ChangeRelationshipState(existingContest, schedule, contest => contest.GameSchedules, EntityState.Deleted);
}
// (4) save the current state
result = this.SaveChanges();
// iterate over new entity relationships and add them
foreach (var newSchedule in schedulesToAdd)
{
// find the schedule
var schedule = this.Context.Schedules.Find(newSchedule.Id);
// (5) mark relationship as added
context.ObjectStateManager.ChangeRelationshipState(existingContest, schedule, contest => contest.GameSchedules, EntityState.Added);
}
// (6) save the current state
this.SaveChanges();
}
keep in mind, you will want to come up with some more elaborate behavior for removing eixsting relationships and adding new ones. This was just an exmaple.