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 = 147,
                    Description = "Expandable Hydration Pack",
                    Price = 19.97M,
                    ImageURL = "/pack147.jpg"
                };
                context.Products.Add(product);
                product = new Product
                {
                    SKU = 178,
                    Description = "Rugged Ranger Duffel Bag",
                    Price = 39.97M,
                    ImageURL = "/pack178.jpg"
                };
                context.Products.Add(product);
                product = new Product
                {
                    SKU = 186,
                    Description = "Range Field Pack",
                    Price = 98.97M,
                    ImageURL = "/noimage.jp"
                };
                context.Products.Add(product);
                product = new Product
                {
                    SKU = 202,
                    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[100];
            byte[] fullBits = new byte[2000];
            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。