entityframework学习笔记--006-表拆分与实体拆分

1.1 拆分实体到多张表

假设你有如下表，如图6-1。Product表用于存储商品的字符类信息，ProductWebInfo用于存储商品的图片，两张表通过SKU关联。现在你想把两张表的信息整合到一个实体类。

图6-1

1.2 使用code first 新建如下的poco实体：

public class Product {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.None)]
        public int SKU { get; set; }
        public string Description { get; set; }
        public decimal Price { get; set; }
        public string ImageURL { get; set; }
    }

1.3 创建一个继承自DbContext的数据访问类“EF6RecipesContext”

public class EF6RecipesContext : DbContext
    {
        public DbSet<Product> Products { get; set; }
        public EF6RecipesContext()
            : base("name=EF6CodeFirstRecipesContext")
        {
        }
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);
            modelBuilder.Entity<Product>()
            .Map(m => {
                m.Properties(p => new { p.SKU, p.Description, p.Price });
                m.ToTable("Product");
            })
            .Map(m => {
                m.Properties(p => new { p.SKU, p.ImageURL });
                m.ToTable("ProductWebInfo");
            });
        }
    }

1.4 测试代码：

 using (var context = new EF6RecipesContext())
             {
                 var product = new Product
                 {
                     SKU = ,
                     Description = "Expandable Hydration Pack",
                     Price = 19.97M,
                     ImageURL = "/pack147.jpg"
                 };
                 context.Products.Add(product);
                 product = new Product
                 {
                     SKU = ,
                     Description = "Rugged Ranger Duffel Bag",
                     Price = 39.97M,
                     ImageURL = "/pack178.jpg"
                 };
                 context.Products.Add(product);
                 product = new Product
                 {
                     SKU = ,
                     Description = "Range Field Pack",
                     Price = 98.97M,
                     ImageURL = "/noimage.jp"
                 };
                 context.Products.Add(product);
                 product = new Product
                 {
                     SKU = ,
                     Description = "Small Deployment Back Pack",
                     Price = 29.97M,
                     ImageURL = "/pack202.jpg"
                 };
                 context.Products.Add(product);
                 context.SaveChanges();
             }
             using (var context = new EF6RecipesContext())
             {
                 foreach (var p in context.Products)
                 {
                     Console.WriteLine("{0} {1} {2} {3}", p.SKU, p.Description,
                     p.Price.ToString("C"), p.ImageURL);
                 }
             }

2.1 分拆一张表到多个实体

你有这样的一张数据库表如图6-2，里面包含经常使用的字符，一些不常用的大字段。为了性能，需要避免每个查询都去加载这些字段。你需要将这张表分拆成两个或是更多的实体。

图6-2

2.2 把上面的表分拆成两个poco实体。

 public class Photograph
     {
         [Key]
         [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
         public int PhotoId { get; set; }
         public string Title { get; set; }
         public byte[] ThumbnailBits { get; set; }
         [ForeignKey("PhotoId")]
         public virtual PhotographFullImage PhotographFullImage { get; set; }
     }

 public class PhotographFullImage
     {
         [Key]
         public int PhotoId { get; set; }
         public byte[] HighResolutionBits { get; set; }
         [ForeignKey("PhotoId")]
         public virtual Photograph Photograph { get; set; }
     }

2.3 创建ef数据访问类，并注册表拆分规则。

 public class EF6RecipesContext2:DbContext
     {
         public EF6RecipesContext2()
             : base("name=EF6CodeFirstRecipesContext")
         {
         }
         public DbSet<Photograph> Photographs { get; set; }
         public DbSet<PhotographFullImage> PhotographFullImages { get; set; }
         protected override void OnModelCreating(DbModelBuilder modelBuilder)
         {
             base.OnModelCreating(modelBuilder);

             modelBuilder.Entity<Photograph>()
             .HasRequired(p => p.PhotographFullImage)
             .WithRequiredPrincipal(p => p.Photograph);

             modelBuilder.Entity<Photograph>().ToTable("Photograph");
             modelBuilder.Entity<PhotographFullImage>().ToTable("Photograph");
         }
     }

2.4 测试代码：

  byte[] thumbBits = new byte[];
             byte[] fullBits = new byte[];
             using (var context = new EF6RecipesContext2())
             {
                 var photo = new Photograph
                 {
                     Title = "My Dog",
                     ThumbnailBits = thumbBits
                 };
                 var fullImage = new PhotographFullImage { HighResolutionBits = fullBits };
                 photo.PhotographFullImage = fullImage;
                 context.Photographs.Add(photo);
                 context.SaveChanges();
             }
             using (var context = new EF6RecipesContext2())
             {
                 //context.Configuration.LazyLoadingEnabled = false;//1 如果关闭演示加载，则注释2必须写上
                 foreach (var photo in context.Photographs)
                 {
                     Console.WriteLine("Photo: {0}, ThumbnailSize {1} bytes",
                         photo.Title, photo.ThumbnailBits.Length);

                     //显式加载存储完整图像的字段
                     //context.Entry(photo).Reference(p => p.PhotographFullImage).Load();//2 这里指定显示加载
                     Console.WriteLine("Full Image Size: {0} bytes",
                         photo.PhotographFullImage.HighResolutionBits.Length);
                 }
             }

输出如下：

Photo: My Dog, Thumbnail Size: 100 bytes
Full Image Size: 2000 bytes

2.5 ef的默认规则是开发延迟加载的，所以在我们获取“Photograph”的时候，其对应的“PhotographFullImage”是不会加载进来的，只有使用到“PhotographFullImage”的属性的时候才会加载进来。