EF中加载实体的方式

 EF中的查询执行时机：
1. foreach进行枚举
2. ToArray、ToList、ToDictionary
3. Linq的一些操作，如First、Any
4. DbSet上的Load操作。DbEntityEntry.Reload和Database.ExecuteSqlCommand

在web application中，每一个请求使用一个context实例；在WPF中，每个form使用一个context实例
context不是线程安全的

加载实体的方式：

1.贪婪加载(eager loading)

2.延迟加载(lazy loading)

3.显示加载(explicit loading)

贪婪加载实现是通过include方法实现的

using (var context = new BloggingContext())
{
    // Load all blogs and related posts
    var blogs1 = context.Blogs
                          .Include(b => b.Posts)
                          .ToList();

    // Load one blogs and its related posts
    var blog1 = context.Blogs
                        .Where(b => b.Name == "ADO.NET Blog")
                        .Include(b => b.Posts)
                        .FirstOrDefault();

    // Load all blogs and related posts 
    // using a string to specify the relationship
    var blogs2 = context.Blogs
                          .Include("Posts")
                          .ToList();

    // Load one blog and its related posts 
    // using a string to specify the relationship
    var blog2 = context.Blogs
                        .Where(b => b.Name == "ADO.NET Blog")
                        .Include("Posts")
                        .FirstOrDefault();
}

延迟加载通过virtual关键字进行实现（当EF框架禁用了延迟加载，这种方式就无效了）

public class Blog
{ 
    public int BlogId { get; set; } 
    public string Name { get; set; } 
    public string Url { get; set; } 
    public string Tags { get; set; } 

    public virtual ICollection<Post> Posts { get; set; } 
}

//禁用延迟加载
public class BloggingContext : DbContext
{
    public BloggingContext()
    {
        this.Configuration.LazyLoadingEnabled = false;
    }
}

当禁用了延迟加载后或不使用延迟加载时，通过显示加载仍然可以获取管理的数据

using (var context = new BloggingContext())
{
    var post = context.Posts.Find(2);

    // Load the blog related to a given post
    context.Entry(post).Reference(p => p.Blog).Load();

    // Load the blog related to a given post using a string 
    context.Entry(post).Reference("Blog").Load();

    var blog = context.Blogs.Find(1);

//一对多时使用Collection
    // Load the posts related to a given blog
    context.Entry(blog).Collection(p => p.Posts).Load();

    // Load the posts related to a given blog 
    // using a string to specify the relationship
    context.Entry(blog).Collection("Posts").Load();
}

//使用Query方式进行一些过滤
using (var context = new BloggingContext())
{
    var blog = context.Blogs.Find(1);

    // Load the posts with the 'entity-framework' tag related to a given blog
    context.Entry(blog)
        .Collection(b => b.Posts)
        .Query()
        .Where(p => p.Tags.Contains("entity-framework")
        .Load();

    // Load the posts with the 'entity-framework' tag related to a given blog 
    // using a string to specify the relationship 
    context.Entry(blog)
        .Collection("Posts")
        .Query()
        .Where(p => p.Tags.Contains("entity-framework")
        .Load();
}