EF Code First 学习笔记:关系

一对多关系

项目中最常用到的就是一对多关系了。Code First对一对多关系也有着很好的支持。很多情况下我们都不需要特意的去配置，Code First就能通过一些引用属性、导航属性等检测到模型之间的关系，自动为我们生成外键。观察下面的类：

public class Destination
{
public int DestinationId { get; set; }
public string Name { get; set; }
public string Country { get; set; }
public string Description { get; set; }
public byte[] Photo { get; set; }
public List<Lodging> Lodgings { get; set; }
}

public class Lodging
{
public int LodgingId { get; set; }
public string Name { get; set; }
public string Owner { get; set; }
public bool IsResort { get; set; }
public decimal MilesFromNearestAirport { get; set; }
public Destination Destination { get; set; }
}

public class Destination
    {
        public int DestinationId { get; set; }
        public string Name { get; set; }
        public string Country { get; set; }
        public string Description { get; set; }
        public byte[] Photo { get; set; }
        public List<Lodging> Lodgings { get; set; }
    }

    public class Lodging
    {
        public int LodgingId { get; set; }
        public string Name { get; set; }
        public string Owner { get; set; }
        public bool IsResort { get; set; }
        public decimal MilesFromNearestAirport { get; set; }
        public Destination Destination { get; set; }
    }

Code First观察到Lodging类中有一个对Destination的引用属性，同时Destination中又有一个集合导航属性Lodgings，因此推测出Destination与Lodging的关系是一对多关系，所以在生成的数据库中为自动为Lodging表生成外键：

其实，只要在一个类中存在引用属性，即：

public class Destination
{
public int DestinationId { get; set; }
public string Name { get; set; }
public string Country { get; set; }
public string Description { get; set; }
public byte[] Photo { get; set; }
}

public class Lodging
{
public int LodgingId { get; set; }
public string Name { get; set; }
public string Owner { get; set; }
public bool IsResort { get; set; }
public decimal MilesFromNearestAirport { get; set; }
public Destination Destination { get; set; }
}

 public class Destination
    {
        public int DestinationId { get; set; }
        public string Name { get; set; }
        public string Country { get; set; }
        public string Description { get; set; }
        public byte[] Photo { get; set; }
    }

    public class Lodging
    {
        public int LodgingId { get; set; }
        public string Name { get; set; }
        public string Owner { get; set; }
        public bool IsResort { get; set; }
        public decimal MilesFromNearestAirport { get; set; }
        public Destination Destination { get; set; }
    } 

或一另一个类中存在导航属性：

public class Destination
{
public int DestinationId { get; set; }
public string Name { get; set; }
public string Country { get; set; }
public string Description { get; set; }
public byte[] Photo { get; set; }
public List<Lodging> Lodgings { get; set; }
}

public class Lodging
{
public int LodgingId { get; set; }
public string Name { get; set; }
public string Owner { get; set; }
public bool IsResort { get; set; }
public decimal MilesFromNearestAirport { get; set; }
}

public class Destination
    {
        public int DestinationId { get; set; }
        public string Name { get; set; }
        public string Country { get; set; }
        public string Description { get; set; }
        public byte[] Photo { get; set; }
        public List<Lodging> Lodgings { get; set; }
    }

    public class Lodging
    {
        public int LodgingId { get; set; }
        public string Name { get; set; }
        public string Owner { get; set; }
        public bool IsResort { get; set; }
        public decimal MilesFromNearestAirport { get; set; }
    } 

Code First都能检测到它们之间一对多的关系，自动生成外键。

 指定外键

当然我们也可以自己在类中增加一个外键。默认情况下，如果你的外键命名是规范的话，Code First会将的该属性设置为外键，不再自动创建一个外键，如：

public class Destination
{
public int DestinationId { get; set; }
public string Name { get; set; }
public string Country { get; set; }
public string Description { get; set; }
public byte[] Photo { get; set; }
public List<Lodging> Lodgings { get; set; }
}

public class Lodging
{
public int LodgingId { get; set; }
public string Name { get; set; }
public string Owner { get; set; }
public bool IsResort { get; set; }
public decimal MilesFromNearestAirport { get; set; }
//外键
public int TargetDestinationId { get; set; }
public Destination Target { get; set; }
}

 public class Destination
    {
        public int DestinationId { get; set; }
        public string Name { get; set; }
        public string Country { get; set; }
        public string Description { get; set; }
        public byte[] Photo { get; set; }
        public List<Lodging> Lodgings { get; set; }
    }

    public class Lodging
    {
        public int LodgingId { get; set; }
        public string Name { get; set; }
        public string Owner { get; set; }
        public bool IsResort { get; set; }
        public decimal MilesFromNearestAirport { get; set; }
        //外键 
        public int TargetDestinationId { get; set; }
        public Destination Target { get; set; }
    } 

规范命名是指符合：命名为“[目标类型的键名]，[目标类型名称]+[目标类型键名称]”，或“[导航属性名称]+[目标类型键名称]”的形式，在这里目标类型就是Destination，相对应的命名就是：DestinationId，DestinationDestinationId，TargetDestinationId

对于命名不规范的列，Code First会怎做呢？

比如我们将外键改为：

public int TarDestinationId { get; set; }

再重新生成数据库：

可以看到Code First没有识别到TarDestinationId是一个外键，于是自己创建了一个外键：Target_DestinationId。这时我们要告诉Code First该属性是一个外键。

使用Data Annotations指定外键：

        [ForeignKey("Target")]
        public int TarDestinationId { get; set; }
        public Destination Target { get; set; }

或

        public int TarDestinationId { get; set; }
        [ForeignKey("TarDestinationId")]
        public Destination Target { get; set; }

注意ForeignKey位置的不同，其后带的参数也不同。这样，生成的数据库就是我们所期望的了。Code First没有再生成别的外键。

用Fluent API指定外键：

modelBuilder.Entity<Lodging>().HasRequired(p => p.Target).WithMany(l => l.Lodgings).HasForeignKey(p => p.TarDestinationId);

对同一个实体多个引用的情况

我们来考虑一下下面的情况：

public class Lodging
{
public int LodgingId { get; set; }
public string Name { get; set; }
public string Owner { get; set; }
public bool IsResort { get; set; }
public decimal MilesFromNearestAirport { get; set; }
public Destination Target { get; set; }
//第一联系人
public Person PrimaryContact { get; set; }
//第二联系人
public Person SecondaryContact { get; set; }
}

public class Person
{
public int PersonID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public List<Lodging> PrimaryContactFor { get; set; }
public List<Lodging> SecondaryContactFor { get; set; }
}

 public class Lodging
    {
        public int LodgingId { get; set; }
        public string Name { get; set; }
        public string Owner { get; set; }
        public bool IsResort { get; set; }
        public decimal MilesFromNearestAirport { get; set; } 
        public Destination Target { get; set; }
        //第一联系人
        public Person PrimaryContact { get; set; }
        //第二联系人
        public Person SecondaryContact { get; set; } 
    } 

    public class Person
    {
        public int PersonID { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public List<Lodging> PrimaryContactFor { get; set; }
        public List<Lodging> SecondaryContactFor { get; set; } 
    }

Lodging(旅店）有两个对Person表的引用，分别是PrimaryContact与SecondaryContact，同时，在Person表中也有对这两个联系人的导航：PrimaryContactFor与SecondaryContactFor。

看看Code First默认会生成怎样的数据库

天哪，竟然生成了四个外键。因为有两套类型一样的导航属性与引用属性，Code First无法确定它们之间的对应关系，就单独为每个属性都创建了一个关系。这肯定不是我们所期望的，为了让Code First知道它们之间的对应关系，在这里要用到逆导航属性来解决。

使用Data Annotations：

       //第一联系人
        [InverseProperty("PrimaryContactFor")] 
        public Person PrimaryContact { get; set; }
        //第二联系人
        [InverseProperty("SecondaryContactFor")] 
        public Person SecondaryContact { get; set; } 

或使用Fluent API：

 modelBuilder.Entity<Lodging>().HasOptional(l => l.PrimaryContact).WithMany(p => p.PrimaryContactFor);
 modelBuilder.Entity<Lodging>().HasOptional(l=>l.SecondaryContact).WithMany(p=>p.SecondaryContactFor);

再重新生成数据库，结果如图：

多对多关系

如果有两个类中，各自都是导航属性指向另一个类，Code First会认为这两个类之间是多对多关系，例如：

public class Lodging
{
public int LodgingId { get; set; }
public string Name { get; set; }
public string Owner { get; set; }
public bool IsResort { get; set; }
public decimal MilesFromNearestAirport { get; set; }
public Destination Target { get; set; }
//第一联系人
public Person PrimaryContact { get; set; }
//第二联系人
public Person SecondaryContact { get; set; }
}

public class Person
{
public int PersonID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public List<Lodging> PrimaryContactFor { get; set; }
public List<Lodging> SecondaryContactFor { get; set; }
}

  public class Activity
     {
         public int ActivityId { get; set; }
         [Required, MaxLength(50)] 
         public string Name { get; set; } 
         public List<Trip> Trips { get; set; }
     }

    public class Trip
    {
        public int TripId{get;set;}
        public DateTime StartDate{get;set;}
        public DateTime EndDate { get; set; }
        public decimal CostUSD { get; set; }
        public byte[] RowVersion { get; set; }
        public List<Activity> Activities { get; set; }
    }

一个Trip类可以有一些Activites日程，而一个Activity日程又可以计划好几个trips(行程)，显然它们之间是多对多的关系。我们看看默认生成的数据库是怎么样的：

可以看到，Code First生成了一张中间表ActivityTrips，将另外两张表的主键都作为外键关联到了中间表上面。中间表中键的命名默认为"[目标类型名称]_[目标类型键名称]".

指定表名

如果我们想指定中间表的名称和键名称，我们可以用Fluent API来配置。

modelBuilder.Entity<Trip>().HasMany(t => t.Activities).WithMany(a => a.Trips).Map(m =>
                {
                    m.ToTable("TripActivities");
                    m.MapLeftKey("TripIdentifier");//对应Trip的主键
                    m.MapRightKey("ActivityId");
                });

或：

 modelBuilder.Entity<Activity>().HasMany(a => a.Trips).WithMany(t => t.Activities).Map(m =>
                {
                    m.ToTable("TripActivities");
                    m.MapLeftKey("ActivityId");//对应Activity的主键
                    m.MapRightKey("TripIdentifier");
                });

一对一关系

如果我们要将两个类配置为一对一关系，则两个类中都要配置相应的引用属性，如：

public class Person
{
public int PersonId { get; set; }
public int SocialSecurityNumber { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
[Timestamp]
public byte[] RowVersion { get; set; }
public PersonPhoto Photo { get; set; }
}

public class PersonPhoto
{
[Key]
public int PersonId { get; set; }
public byte[] Photo { get; set; }
public string Caption { get; set; }
public Person PhotoOf { get; set; }
}

 public class Person
    {
        public int PersonId { get; set; }
        public int SocialSecurityNumber { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        [Timestamp]
        public byte[] RowVersion { get; set; }
        public PersonPhoto Photo { get; set; }
    }

    public class PersonPhoto
    {
        [Key]
        public int PersonId { get; set; }
        public byte[] Photo { get; set; }
        public string Caption { get; set; }
        public Person PhotoOf { get; set; }
    }

我们为一个(Person)对应着一张相片(PersonPhoto),但如果根据这样的模型生成数据库为报错：

无法确定类型“BreakAway.PersonPhoto”与“BreakAway.Person”之间的关联的主体端。必须使用关系 Fluent API 或数据注释显式配置此关联的主体端

因为Code First无法确认哪个是依赖类，必须使用Fluent API或Data Annotations进行显示配置。

使用Data Annotations：

public class Person
{
public int PersonId { get; set; }
public int SocialSecurityNumber { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
[Timestamp]
public byte[] RowVersion { get; set; }
public PersonPhoto Photo { get; set; }
}

public class PersonPhoto
{
[Key, ForeignKey("PhotoOf")]
public int PersonId { get; set; }
public byte[] Photo { get; set; }
public string Caption { get; set; }
public Person PhotoOf { get; set; }
}

public class Person
    {
        public int PersonId { get; set; }
        public int SocialSecurityNumber { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        [Timestamp]
        public byte[] RowVersion { get; set; }
        public PersonPhoto Photo { get; set; }
    }

    public class PersonPhoto
    {
        [Key, ForeignKey("PhotoOf")]
        public int PersonId { get; set; }
        public byte[] Photo { get; set; }
        public string Caption { get; set; }
        public Person PhotoOf { get; set; }
    }

使用Fluent API：

modelBuilder.Entity<PersonPhoto>().HasRequired(p => p.PhotoOf).WithOptional(p => p.Photo);

 注意：PersonPhoto表中的PersonId既是外键也必须是主键

public class User
{
[Key]
public int UserId { get; set; }
public string UserName { get; set; }
public string Password { get; set; }
public bool IsActive { get; set; }
public System.DateTime CreateDate { get; set; }
public virtual Student Student { get; set; }
public virtual ICollection<Teacher> Teachers { get; set; }
public virtual UserDetail UserDetail { get; set; }
}

public class Teacher
{
[Key,ForeignKey("User")]
public int UserId { get; set; }
[Key, ForeignKey("Class")]
public int ClassId { get; set; }

[Key, ForeignKey("Course")]
public string CourseId { get; set; }

[Key, ForeignKey("Role")]
public int RoleId { get; set; }

public virtual Class Class { get; set; }
public virtual Course Course { get; set; }
public virtual Role Role { get; set; }
public virtual User User { get; set; }
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{

base.OnModelCreating(modelBuilder);
//一对一
modelBuilder.Entity<UserDetail>().HasRequired(p => p.User).WithOptional(p => p.UserDetail);
modelBuilder.Entity<Student>().HasRequired(p => p.User).WithOptional(p => p.Student);
//一对多
modelBuilder.Entity<Teacher>().HasKey(t => new { t.UserId, t.CourseId,t.ClassId,t.RoleId });
modelBuilder.Entity<Teacher>().HasRequired(p => p.User).WithMany(l => l.Teachers).HasForeignKey(p => p.UserId);
modelBuilder.Entity<Teacher>().HasRequired(p => p.Class).WithMany(l => l.Teachers).HasForeignKey(p => p.ClassId);
modelBuilder.Entity<Teacher>().HasRequired(p => p.Course).WithMany(l => l.Teachers).HasForeignKey(p => p.CourseId);
modelBuilder.Entity<Teacher>().HasRequired(p => p.Role).WithMany(l => l.Teachers).HasForeignKey(p => p.RoleId);
}