EF Code First 在多对多中

Question

我有这些课程：

public class User
{
 public int Id {get; set;}
 public string Name {get; set;}
}

数据库表 - 用户

public class Pet
{
 public int Id {get; set;}
 public string Name {get; set;}
}

数据库表 - 宠物

public class UsersPets
{
 public int UserId {get; set;}
 public int PetId {get; set;}
}

数据库表 - users_pets

到现在我可以用 linq 获取用户的宠物。但是如何在 EF Code First 中自动映射 User.Pets 而无需额外的 Linq 查询？

Answer 1

你不能把你的课程改成：

public class User
{
    public User(){
        Pets = new HashSet<Pet>();
    }

    public int Id {get; set;}
    public string Name {get; set;}

    public ICollection<Pet> Pets;
}

public class Pet
{
    public Pet(){
        Users = new HashSet<User>();
    }

    public int Id {get; set;}
    public string Name {get; set;}

    public ICollection<User> Users;
}

Answer 2

EF 为您创建此表，您不应该在模型中执行此操作。所以：

public class User
{
 public int Id {get; set;}
 public string Name  {get; set;}
public ICollection<Pet> Pets  {get; set;}
} 

public class Pet
{
 public int Id {get; set;}
 public string Name {get; set;}
}

将在数据库中创建附加表，您可以在代码中访问 User 实体的 Pets 集合。

Answer 3

对于简单的多对多关系，您不需要额外的类，您只需向 User 和 Pet 类添加两个属性：

public class User
{
    public int Id {get; set;}
    public string Name {get; set;}

    public virtual ICollection<Pet> Pets { get; set; }

    public User
    {
        Pets = new List<Pet>();
    }
}

public class Pet
{
    public int Id {get; set;}
    public string Name {get; set;}

    public virtual ICollection<User> Users { get; set; }

    public Pet
    {
        Users = new List<User>();
    }
}

注意Pets 和Users 集合是virtual。这将启用延迟加载，以防止在您不需要用户的宠物时加载它们。

// Pets not loaded
var user = db.Users.Find(1);

// This loads the pets for the user (lazy loading)
foreach (var pet in user.Pets)
{
    ...
}

// This immediately loads the pets for the user (eager loading)
var user2 = db.Users.Include(u => u.Pets).SingleOrDefault(u => u.Id == 2);