Entity Framework - many to many with one primary - entity-framework

Question

I have a Channel domain model as:
public class Channel
{
public string Id {get; set;}
public List<ChannelUser> ChannelUsers {get; set;}
}
and a ChannelUser class as follows (I have additional meta data on the table, removed here for brevity, which is why I used a specfic class to represent the many-to-many relationship).
public class ChannelUser
{
public string ChannelId {get; set;}
public string UserId {get; set;}
}
The primary key for ChannelUser is (ChannelId, UserId).
One (and only one) of these ChannelUsers can be the Owner of the Channel.
Theoretically, a User can be the Owner of many channels (but obviously a ChannelUser can only be the owner of the channel it is associated to).
I want to enforce that at database level rather than having a "IsOwner" property on ChannelUser and using business logic to ensure that the flag is only set once per channel. I want to also enforce that the ChannelOwner is one of the ChannelUsers and therefore don't want a relationship directly from Channel > User but keep it as Channel > ChannelUser.
Therefore I have updated Channel class as follows:
public class Channel
{
public string Id {get; set;}
public List<ChannelUser> {get; set;}
public ChannelUser Owner {get; set;}
}
And added the following fluent API expressions:
modelBuilder.Entity<ChannelUser>().HasKey(cu => new { cu.ChannelId, cu.UserId });
modelBuilder.Entity<ChannelUser>().HasOne<Channel>(x => x.Channel).WithMany(x => x.ChannelUsers)
.OnDelete(DeleteBehavior.NoAction);
modelBuilder.Entity<ChannelUser>().HasOne<User>(x => x.User).WithMany(x => x.UserChannels)
.OnDelete(DeleteBehavior.NoAction);
When adding migration this results in:
Both relationships between 'ChannelUser.Channel' and 'Channel.ChannelUsers' and between 'ChannelUser' and 'Channel.Owner' could use {'ChannelId'} as the foreign key. To resolve this configure the foreign key properties explicitly on at least one of the relationships.
Which I understand but I think this is what is desired. Because ChannelUser.ChannelId should always be the same Channel.Id for both relationships.
Please can anyone suggest how I can craft this relationship or an alternative approach that still enforces the many to many and the one to one relationships between the tables?

Answer 1

This isn't ideal approach but can be done, have a look at this ER diagram :
Ideal approach would be having proper Entity Mapping like :
User Class :
public class User
{
public string Id {get; set;}
public virtual List<Channel> Channels {get; set;}
// Other properties....
}
Channel Class :
public class Channel
{
public string Id {get; set;}
public List<User> Users {get; set;}
public User Owner {get; set;}
public string? OwnerId { get; set; }
public User Owner { get; set; }
}
ChannelUser Class to join the two:
public class ChannelUser
{
public string Id {get; set;}
public string ChannelId {get; set;}
public string UserId {get; set;}
}
Mapping :
modelBuilder.Entity<User>()
.HasMany<Channel>(user => user.channels)
.WithMany(channel => channel.users)
.Map(cu =>
{
cu.MapLeftKey("UserId");
cu.MapRightKey("ChannelId");
cu.ToTable("ChannelUser");
});
// configure one-to-many relationship for ownership
modelBuilder.Entity<Channel>()
.HasRequired<User>(c => c.User)
.WithMany(u => u.ChannelId )
.HasForeignKey<int>(c => c.UserId);
Responsibility of checking if user exists should be fairly easy in controller code after normalizing this, or we can use DB procedure, trigger to create constrain that checks if given owner ID is indeed in ChannelUser table for given channel.