EF 使用笔记

1 事务：一 次性保存全部的变化

using (JDDbContext context = new JDDbContext())
 {
          context.Database.Log += c => Console.WriteLine(c);

          User user2 = context.Users.Find(2);
          user2.Name = user2.Name + "123";

         User user7 = context.Users.Find(7);
         user7.Name = user7.Name + "123123123123";
         context.SaveChanges();
    
}

   2.1  context更新时保存全部变化，不能全局都是一个context

   2.2 不同的context 也不能join的，除非都load到内存之后再去操作
   2.3 context是一个数据库连接+内存对象，资源开销  一次请求，就是一个context，也不排除可能多个；
   2.4更多的时候，是一个service一个context实例，多个请求 /多线程 最好是不同的context

   using (JDDbContext context1 = new JDDbContext())

   using (JDDbContext context2 = new JDDbContext())
  {
        context1.Database.Log += c => Console.WriteLine(c);
        context2.Database.Log += c => Console.WriteLine(c);

          var list = from u in context1.Users
          join c in context2.Categories on u.CompanyId equals c.Id
        into ucList
         from uc in ucList.DefaultIfEmpty()
          where new int[] { 1, 2, 3, 4, 6, 7, 10 }.Contains(u.Id)
         select new
        {
              Account = u.Account,
             Pwd = u.Password
         };
          foreach (var user in list)
         {
               Console.WriteLine("{0} {1}", user.Account, user.Pwd);
         }
}

 3 本地缓存   Where查询不缓存；Find先在本地缓存查找(可能脏读，性能好处)

using (JDDbContext context = new JDDbContext())

{
     context.Database.Log += c => Console.WriteLine(c);
     var list = context.Users.Where(u => u.Id < 5).ToList();
    Console.WriteLine("*****************0*****************");
    var user1 = context.Users.Find(2);
    Console.WriteLine("*****************1*****************");
    var user2 = context.Users.Where(u => u.Id == 2).ToList();
    Console.WriteLine("*****************2*****************");
    var user3 = context.Users.Find(2);
    Console.WriteLine("*****************3*****************");
    var user4 = context.Users.Where(u => u.Id == 2).ToList();
    Console.WriteLine("*****************4*****************");
}

4  本地缓存 AsNoTracking() 表示不跟踪，不在内存做clone(增删改不行，有性能提升)

 using (JDDbContext context = new JDDbContext())
{
    context.Database.Log += c => Console.WriteLine(c);
    var list1 = context.Users.Where(u => u.Id > 5).AsNoTracking().ToList();
    Console.WriteLine("*****************list1*****************");
    var user1 = context.Users.Find(16);
    Console.WriteLine("*****************1*****************");
    var list2 = context.Users.Where(u => u.Id > 5).ToList();
    Console.WriteLine("******************list2****************");
    var userx = context.Users.Find(16);
    Console.WriteLine("*****************1*****************");
    var list3 = context.Users.Where(u => u.Id > 5).ToList();
    Console.WriteLine("******************list3****************");
    var list4 = context.Users.Where(u => u.Id > 5).ToList();
    Console.WriteLine("*****************list4*****************");
}

5  导航属性延迟加载/预先加载/显示加载

Console.WriteLine("******************************************");
using (JDDbContext context = new JDDbContext())
{
    context.Database.Log += c => Console.WriteLine(c);
    var companyList = context.Set<Company>().Where(c => c.Id > 0);
    //var companyList = context.Set<Company>().Where(c => c.Id > 0).ToList();//.ToList()直接Company都加载过来

    foreach (var company in companyList)
    {
        Console.WriteLine("Company id={0} name={1}", company.Id, company.Name);
    }
 }

//1 延迟加载 每次使用子表时再查询
//2 放弃子表查询
//3 Include一次性主从表
//4 放弃子表查询，又去显示获取某个数据
//主流选择会去导航，内置的方便；innor join也能完成

Console.WriteLine("******************************************");

using (JDDbContext context = new JDDbContext())
{
    context.Database.Log += c => Console.WriteLine(c);
    //实体类型包含其它实体类型（POCO类）的属性（也可称为导航属性），且同时满足如下条件即可实列延迟加载，
    //1.该属性的类型必需为public且不能为Sealed；
    //2.属性标记为Virtual
    context.Configuration.LazyLoadingEnabled = true;//默认是true 针对导航属性的
    var companyList = context.Set<Company>().Where(c => c.Id > 0);
    foreach (var company in companyList)
    {
        Console.WriteLine("Company id={0} name={1}", company.Id, company.Name);
        foreach (var item in company.Users)//这个时候才去数据库查询user
        {
            Console.WriteLine("User name={0}", item.Name);
         }
        }
    }

//不延迟加载,不会再次查询了  
 Console.WriteLine("******************************************");
using (JDDbContext context = new JDDbContext())
{
    context.Database.Log += c => Console.WriteLine(c);
    context.Configuration.LazyLoadingEnabled = false
    var companyList = context.Set<Company>().Where(c => c.Id > 0);
    foreach (var company in companyList)
    {
        Console.WriteLine("Company id={0} name={1}", company.Id, company.Name);
        foreach (var item in company.Users)//这个时候才不去数据库查询了，所以用户全是空的了
        {
            Console.WriteLine("User name={0}", item.Name);
        }
    }
}

Console.WriteLine("******************************************");

//不延迟加载，指定Include，一次性加载出来  
using (JDDbContext context = new JDDbContext())
{
    context.Database.Log += c => Console.WriteLine(c);
    //context.Configuration.LazyLoadingEnabled = false;
    var companyList = context.Set<Company>().Include("Users").Where(c => c.Id > 0);
    foreach (var company in companyList)
    {
        Console.WriteLine("Company id={0} name={1}", company.Id, company.Name);
        foreach (var item in company.Users)
        {
            Console.WriteLine("User name={0}", item.Name);
        }
    }
}

//LoadProperty 手动加载   放弃子表查询，又去显示获取某个数据

using (JDDbContext context = new JDDbContext())
{
    ontext.Database.Log += c => Console.WriteLine(c);
    context.Configuration.LazyLoadingEnabled = false;//不延迟加载，指定Include，一次性加载出来
    var companyList = context.Set<Company>().Where(c => c.Id > 0);
    foreach (var company in companyList)
    {
            Console.WriteLine("Company id={0} name={1}", company.Id, company.Name);
            context.Entry<Company>(company).Collection(c => c.Users).Load();//集合显示加载
            //context.Entry<Company>(company).Reference(c => c.User).Load();//单个属性用
            foreach (var item in company.Users)
            {
                    Console.WriteLine("User name={0}", item.Name);
            }
        }
}
6   Linq to  Sql 

   //In 写法

     var list = from u in dbContext.Users
    where new int[] { 1, 2, 3, 5, 7, 8, 9, 10, 11, 12, 14 }.Contains(u.Id)
    select u;
   

    var list = dbContext.Users.Where(u => new int[] { 1, 2, 3, 5, 7, 8, 9, 10, 11, 12, 14 }.Contains(u.Id)).OrderBy(u => u.Id)
    .Select(u => new
    {
        Account = u.Account,
        Pwd = u.Password
    }).Skip(3).Take(5);  

   //like  写法  
    var list = dbContext.Users.Where(u => u.Name.StartsWith("小") && u.Name.EndsWith("新"))
    .Where(u => u.Name.EndsWith("新"))
    .Where(u => u.Name.Contains("小新"))
    .Where(u => u.Name.Length < 5)
    .OrderBy(u => u.Id);  

  //inner join   写法    
    var list = from u in dbContext.Users
    join c in dbContext.Companies on u.CompanyId equals c.Id
    where new int[] { 1, 2, 3, 4, 6, 7, 10 }.Contains(u.Id)
    select new
    {
            Account = u.Account,
            Pwd = u.Password,
            CompanyName = c.Name
    };

    //left join   写法      
    var list = from u in dbContext.Users
    oin c in dbContext.Categories on u.CompanyId equals c.Id
    into ucList
    from uc in ucList.DefaultIfEmpty()
    where new int[] { 1, 2, 3, 4, 6, 7, 10 }.Contains(u.Id)
    select new
    {
        Account = u.Account,
        Pwd = u.Password
    };

    //事务  写法        
  using (JDDbContext dbContext = new JDDbContext())

  {
        DbContextTransaction trans = null;
        try
        {
            trans = dbContext.Database.BeginTransaction();
            string sql = "Update [User] Set Name='小新' WHERE Id=@Id";
            SqlParameter parameter = new SqlParameter("@Id", 1);
            dbContext.Database.ExecuteSqlCommand(sql, parameter);
            trans.Commit();
        }
        catch (Exception ex)
        {
            if (trans != null)
            trans.Rollback();
             throw ex;
        }
        finally
        {
            trans.Dispose();
        }
    }

         //SQL  写法        
        DbContextTransaction trans = null;
        try
        {
            trans = dbContext.Database.BeginTransaction();
            string sql = "SELECT * FROM [User] WHERE Id=@Id";
            SqlParameter parameter = new SqlParameter("@Id", 1);
            List<User> userList = dbContext.Database.SqlQuery<User>(sql, parameter).ToList<User>();
            trans.Commit();
        }
        catch (Exception ex)
        {
            if (trans != null)
            trans.Rollback();
            throw ex;
        }
        finally
        {
                trans.Dispose();
        }
    }

7  其他

  using (JDDbContext context = new JDDbContext())
{
        context.Database.Log += c => Console.WriteLine($"sql：{c}");
         context.Users.Add(userNew);
        context.SaveChanges();//id自动赋值
        userNew.Name += "1";
        context.SaveChanges();//更新 删除都没问题
}

//context实例会跟踪数据(插入/查询) 修改过就会有个状态变化
using (JDDbContext context = new JDDbContext())
{
        context.Database.Log += c => Console.WriteLine($"sql：{c}");
        context.Users.Attach(userNew);//直接保存无效 Attach一下,附加之后就可以监控
        userNew.Name += "2";
        context.SaveChanges();
}

using (JDDbContext context = new JDDbContext())
{
        context.Database.Log += c => Console.WriteLine($"sql：{c}");
        userNew = context.Users.Find(userNew.Id);
        userNew.Name += "3";
        context.SaveChanges();
}

using (JDDbContext context = new JDDbContext())
{
        context.Database.Log += c => Console.WriteLine($"sql：{c}");
        userNew.Name += "2";
        context.Users.Attach(userNew);//直接保存无效 Attach一下,附加之后就可以监控
        //context.Entry<User>(userNew).State = EntityState.Modified;
        //告诉context数据改了
        context.Entry<User>(userNew).Property<string>("Name").IsModified = true;
        //告诉context说 name属性更改了
        context.SaveChanges();
}

//按需更新:DTO对象--Find---赋值--save；传递json--解析json
{
        //Update A Set Name=1 Where Name like ''
        //EF不支持， 要么直接sql；要么表达式目录树解析sql 然后执行sql；
}