Entity Framework Code First -- 延迟加载和预先加载

还是以这两个表为例子

country包含零个或多个city， 这个外键关系是我后来加上去，原来没有。 然后再用Power Tool逆向， 产生如下代码

   1:  using System.ComponentModel.DataAnnotations.Schema;

   2:  using System.Data.Entity.ModelConfiguration;

   3:   

   4:  namespace EFEntity.Models.Mapping

   5:  {

   6:      public class cityMap : EntityTypeConfiguration<city>

   7:      {

   8:          public cityMap()

   9:          {

  10:              // Primary Key

  11:              this.HasKey(t => t.ID);

  12:   

  13:              // Properties

  14:              this.Property(t => t.Name)

  15:                  .IsRequired()

  16:                  .HasMaxLength(35);

  17:   

  18:              this.Property(t => t.CountryCode)

  19:                  .IsRequired()

  20:                  .HasMaxLength(3);

  21:   

  22:              this.Property(t => t.District)

  23:                  .IsRequired()

  24:                  .HasMaxLength(20);

  25:   

  26:              // Table & Column Mappings

  27:              this.ToTable("city", "world");

  28:              this.Property(t => t.ID).HasColumnName("ID");

  29:              this.Property(t => t.Name).HasColumnName("Name");

  30:              this.Property(t => t.CountryCode).HasColumnName("CountryCode");

  31:              this.Property(t => t.District).HasColumnName("District");

  32:              this.Property(t => t.Population).HasColumnName("Population");

  33:   

34: // Relationships //这里加了一个关系 35: this.HasRequired(t => t.country) //这个指向city 模型的 public virtual country country { get; set; } 36: .WithMany(t => t.cities) //这个指向country模型的  public virtual ICollection<city> cities { get; set; } 37: .HasForeignKey(d => d.CountryCode); //这个指向city模型的public string CountryCode { get; set; }

  38:   

  39:          }

  40:      }

  41:  }

以上是city映射， 下面是country映射

using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Entity.ModelConfiguration;

namespace EFEntity.Models.Mapping
{
    public class countryMap : EntityTypeConfiguration<country>
    {
        public countryMap()
        {
            // Primary Key
            this.HasKey(t => t.Code);

            // Properties
            this.Property(t => t.Code)
                .IsRequired()
                .HasMaxLength(3);

            this.Property(t => t.Name)
                .IsRequired()
                .HasMaxLength(52);

            this.Property(t => t.Continent)
                .IsRequired()
                .HasMaxLength(65532);

            this.Property(t => t.Region)
                .IsRequired()
                .HasMaxLength(26);

            this.Property(t => t.LocalName)
                .IsRequired()
                .HasMaxLength(45);

            this.Property(t => t.GovernmentForm)
                .IsRequired()
                .HasMaxLength(45);

            this.Property(t => t.HeadOfState)
                .HasMaxLength(60);

            this.Property(t => t.Code2)
                .IsRequired()
                .HasMaxLength(2);

            // Table & Column Mappings
            this.ToTable("country", "world");
            this.Property(t => t.Code).HasColumnName("Code");
            this.Property(t => t.Name).HasColumnName("Name");
            this.Property(t => t.Continent).HasColumnName("Continent");
            this.Property(t => t.Region).HasColumnName("Region");
            this.Property(t => t.SurfaceArea).HasColumnName("SurfaceArea");
            this.Property(t => t.IndepYear).HasColumnName("IndepYear");
            this.Property(t => t.Population).HasColumnName("Population");
            this.Property(t => t.LifeExpectancy).HasColumnName("LifeExpectancy");
            this.Property(t => t.GNP).HasColumnName("GNP");
            this.Property(t => t.GNPOld).HasColumnName("GNPOld");
            this.Property(t => t.LocalName).HasColumnName("LocalName");
            this.Property(t => t.GovernmentForm).HasColumnName("GovernmentForm");
            this.Property(t => t.HeadOfState).HasColumnName("HeadOfState");
            this.Property(t => t.Capital).HasColumnName("Capital");
            this.Property(t => t.Code2).HasColumnName("Code2");
        }
    }
}

测试代码--延迟加载：

using (var context = new worldContext())
           {
               var country = from d in context.countries
                           where d.Name == "United States"
                           select d;
               var countryUS = country.Single();
               if (countryUS == null) return;
               foreach (var city in countryUS.cities)
               {
                   Console.WriteLine(city.Name);
               }
               Console.Read();

 

测试代码--预先加载：

            using (var context = new worldContext())
            {
                var allCountries = context.countries.Include(p => p.cities);
                foreach (var country in allCountries)
                {
                    Console.WriteLine(country.Name);
                    foreach (var city in country.cities)
                    {
                        Console.WriteLine("__" + city.Name);
                    }
                }
                Console.Read();
            }