EntityFramework Core延迟加载(LazyLoad)

文章目录

• 1、延迟加载介绍
• 2、使用延迟加载
• 2.1 使用 Microsoft.EntityFrameworkCore.Proxies
• 2.2使用ILazyLoader注入到构造中
• 2.3使用Action

1、延迟加载介绍

延迟加载又叫惰性加载（Lazy Loading）：即在需要或者使用的时候加载数据。

此功能是在EF Core 2.1中引入的。

2、使用延迟加载

2.1 使用 Microsoft.EntityFrameworkCore.Proxies

直接重写OnConfiguring方法配置UseLazyLoadingProxies()

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    => optionsBuilder
        .UseLazyLoadingProxies()
        .UseSqlServer(myConnectionString);

或者在Configing方法中添加

services.AddDbContext<UserContext>(
    b => b.UseLazyLoadingProxies()
          .UseSqlServer(connectionString));

2.2使用ILazyLoader注入到构造中

• 定义实体,并构造注入ILazyLoader

public class Blog
{
    private ICollection<Post> _posts;
    public Blog()
    {
    }
    private Blog(ILazyLoader lazyLoader)
    {
        LazyLoader = lazyLoader;
    }
    private ILazyLoader LazyLoader { get; set; }
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<Post> Posts
    {
        get => LazyLoader.Load(this, ref _posts);
        set => _posts = value;
    }
}
public class Post
{
    private Blog _blog;
    public Post()
    {
    }
    private Post(ILazyLoader lazyLoader)
    {
        LazyLoader = lazyLoader;
    }
    private ILazyLoader LazyLoader { get; set; }
    public int Id { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }
    public Blog Blog
    {
        get => LazyLoader.Load(this, ref _blog);
        set => _blog = value;
    }
}

注意：构造注入的方法是私有的，因为ILazyLoader只和EFCore进行耦合。

2.3使用Action<object, string>让上面的ILazyLoader解耦

• 使用注入委托的方式进行解耦

  public class Blog
  {
      private ICollection<Post> _posts;
      public Blog()
      {
      }
      private Blog(Action<object, string> lazyLoader)
      {
          LazyLoader = lazyLoader;
      }
      private Action<object, string> LazyLoader { get; set; }
      public int Id { get; set; }
      public string Name { get; set; }
      public ICollection<Post> Posts
      {
          get => LazyLoader.Load(this, ref _posts);
          set => _posts = value;
      }
  }
  public class Post
  {
      private Blog _blog;
      public Post()
      {
      }
      private Post(Action<object, string> lazyLoader)
      {
          LazyLoader = lazyLoader;
      }
      private Action<object, string> LazyLoader { get; set; }
      public int Id { get; set; }
      public string Title { get; set; }
      public string Content { get; set; }
      public Blog Blog
      {
          get => LazyLoader.Load(this, ref _blog);
          set => _blog = value;
      }
  }
  

• 扩展Load方法

  public static class PocoLoadingExtensions
  {
      public static TRelated Load<TRelated>(
          this Action<object, string> loader,
          object entity,
          ref TRelated navigationField,
          [CallerMemberName] string navigationName = null)
          where TRelated : class
      {
          loader?.Invoke(entity, navigationName);
  
          return navigationField;
      }
  }
  

3、处理序列化时的循环依赖

添加AddJsonOptions

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc()
        .AddJsonOptions(
            options => options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
        );
}

或者使用 [JsonIgnore] 标记属性表示序列化的时候不进行序列化。