EntityFramework自动创建多对多关系

Question

Does Entity Framework create Many-Many relationships automatically and if so how?

I am trying this below:

public DbSet<User> Users { get; set; }
public DbSet<Sport> Sports { get; set; }
public DbSet<Team> Teams { get; set; }

• I have many Sports
• I have many Teams in a Sport
• I have many Users in a Team

Classes:

Sport

public int ID { get; set; }
public List<Team> Teams { get; set; }

Team

public int ID { get; set; }
public List<User> Members { get; set; }
public int SportID { get; set; }

User

 public int UserID { get; set; }

Issue:

So it creates the tables Users, Sports, Teams fine with their foreign key associations to each other. The problem is that these tables are not normalized and have redundant data. Also, when trying to query for members in the Teams table, I always get a null Members List!

For Example, Users has a column Team_ID so it creates a new User record when added to a team which is bad!

Shouldn't Entity Framework create a Many-Many relationship table automatically? So that it has columns |UserID|SportID|TeamID|

Answer 1

Users has a column Team_ID

Here is the problem. EF created a column in User table because you told EF to do so. You should make it many-to-many.

public class User
{
   public int UserId { set;get;}
   public ICollection<Team> Teams { set; get; }
}
public class Sport
{
    public int Id { set; get; }
    public string Name { set; get; }
    public IEnumerable<Team> Teams { set; get; }
}

public class Team
{
    public int TeamId { get; set; }
    public List<User> Users { get; set; }
    public int SportId { set; get; }
}

Now using fluent configuration, you can instruct EF to create a junction table for the many to many relation ship. You can do this in the OnModelCreating method of your dbcontext class.

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<User>()
                .HasMany(d => d.Teams)
                .WithMany(f => f.Users)
                .Map(w => w.ToTable("UserTeam")
                           .MapLeftKey("UserId")
                           .MapRightKey("TeamId"));

}

This will establish a many- to many relationship between User and Team and create a fourth table which has a UserId and TeamId column.