Entity Framework 的懒加载、预先加载、显示加载

1.新建两个实体，一个班级有多个学生

public class Student
{
    public int StudentId { get; set; }
    public string StudentName { get; set; }

    public Grade Grade { get; set; }

}

public class Grade
{
    public int GradeId { get; set; }
    public string GradeName { get; set; }
    public string Section { get; set; }

    public virtual ICollection<Student> Students { get; set; }
}

2.控制台代码

 using (var db = new MyDbContext())
 {

     db.Database.Log = (sql) => Console.WriteLine(sql);

     //懒加载 foreach时才会生成Sql语句，导航属性也是如此
     foreach (var item in db.Grade)
     {
         Console.WriteLine($"班级名称：{item.GradeName}");
         foreach (var subItem in item.Students)
         {
             Console.WriteLine($"学生名称：{subItem.StudentName}");
         }
     }

     //预先加载 foreach时生成查询一个包含导航属性的查询语句，返回一个大实体，foreach students的不再查询数据库
     var grades = db.Grade.Include(g => g.Students);
     foreach (var item in grades)
     {
         Console.WriteLine($"班级名称：{item.GradeName}");
         foreach (var subItem in item.Students)
         {
             Console.WriteLine($"学生名称：{subItem.StudentName}");
         }
     }

     //预先加载过滤
     db.Configuration.LazyLoadingEnabled = false;
     var filterGrades = db.Grade
         .Select(g => new
         {
             g,
             Students = g.Students.Where(s => s.StudentId > 0)
         })
         .AsEnumerable()
         .Select(x => x.g);
     foreach (var item in filterGrades)
     {
         Console.WriteLine($"班级名称：{item.GradeName}");
         if (item.Students != null)
         {
             foreach (var subItem in item.Students)
             {
                 Console.WriteLine($"学生名称：{subItem.StudentName}");
             }
         }
     }

     //显示加载 可以显示的控制加载的导航属性
     db.Configuration.LazyLoadingEnabled = false;
     var grade = db.Grade.First();
     db.Entry(grade).Collection(c => c.Students).Take(1).Load();
     Console.WriteLine($"班级名称：{grade.GradeName}");
     foreach (var item in grade.Students)
     {
         Console.WriteLine($"学生名称：{item.StudentName}");
     }

 }