使用Entity Framework Core查询多对多相关数据

Question

I have two entities that have a many-to-many relationship. I also have an entity between then that holds the two ID's. I want to load only the two main entities.

My entities:

public class User 
{
    public long Id { get; set; }
    public string Email { get; set; }
    public virtual ICollection<UserRole> UserRoles { get; set; }
}

public class Role 
{
    public long Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<UserRole> UserRoles { get; set; }
}

public class UserRole 
{
    public long UserId { get; set; }
    public User User { get; set; }
    public long RoleId { get; set; }
    public Role Role { get; set; }
}

Expected JSON result:

{
  "Email": ...
  "Roles": [
    {"Name": ...},
    {"Name": ...},
    ...
  ] 
}

Answer 1

EF Core doesn't technically support M2M relationships. You can fake said support via a "join" entity, which connects the two sides of the relationship, as you have here with UserRole . There's no way around this, unfortunately, so if you don't want to see the UserRole relationship in your results, you'll have to manually reshape the data. For example:

var users = await _context.Users.Select(u => new
{
    Email = u.Email,
    Roles = u.UserRoles.Select(r => r.Role)
}).ToListAsync();

Pro Tip: You don't need Include if you're selecting from relationships. EF Core is smart enough to issue joins in order to fill the data.