entity-framework - .Net Entity Framework SaveChanges 正在添加而无需添加方法

Question

我是 Entity Framework 的新手，我对 savechanges 的工作原理感到非常困惑。我的示例中可能有很多代码可以改进，但这就是我遇到的问题。

用户输入一堆选择。我确保用户尚未输入这些选择。 然后我将选择添加到数据库中。

       var db = new myModel()
       var predictionArray = ticker.Substring(1).Split(','); // Get rid of the initial comma.

        var user = Membership.GetUser();
        var userId = Convert.ToInt32(user.ProviderUserKey);

        // Get the member with all his predictions for today.
        var memberQuery = (from member in db.Members
                         where member.user_id == userId
                         select new
                                    {
                                        member,
                                        predictions = from p in member.Predictions
                                                     where p.start_date == null
                                                     select p

                                    }).First();

        // Load all the company ids.
        foreach (var prediction in memberQuery.predictions)
        {
            prediction.CompanyReference.Load();
        }

 var picks = from prediction in predictionArray
                          let data = prediction.Split(':')
                          let companyTicker = data[0]
                          where !(from i in memberQuery.predictions
                                 select i.Company.ticker).Contains(companyTicker)
                          select new Prediction
                          {
                              Member = memberQuery.member,
                              Company = db.Companies.Where(c => c.ticker == companyTicker).First(),
                              is_up = data[1] == "up",  // This turns up and down into true and false.
                          };


    // Save the records to the database.
    // HERE'S THE PART I DON'T UNDERSTAND.
    // This saves the records, even though I don't have db.AddToPredictions(pick) 
            foreach (var pick in picks)
            {
                    db.SaveChanges();
            }

// This does not save records when the db.SaveChanges outside of a loop of picks.
 db.SaveChanges();
 foreach (var pick in picks)
            {

            }

// This saves records,  but it will insert all the picks exactly once no matter how many picks you have.  
//The fact you're skipping a pick makes no difference in what gets inserted.
            var counter = 1;
            foreach (var pick in picks)
            {
                if (counter == 2)
                {
                    db.SaveChanges();
                }
                counter++;
            }

我已经测试过，SaveChanges 甚至不必在循环中。 下面的代码也有效。

foreach (var pick in picks)
            {
                   break;
            }

db.SaveChanges()

显然，我不明白上下文中发生了什么。我猜我已经以某种方式将我的新选择加载为待处理的更改，但即使这是真的，我也不明白我必须循环它们才能保存更改。

有人可以向我解释一下吗？

这是根据克雷格的回复更新的工作代码: 1) 删除 Type，然后循环结果并填充新对象。

var  picks = (from prediction in predictionArray
                        let data = prediction.Split(':')
                        let companyTicker = data[0]
                        where !(from i in memberQuery.predictions
                                select i.Company.ticker).Contains(companyTicker)
                        select new //NO TYPE HERE
                        {
                            Member = memberQuery.member,
                            Company = db.Companies.Where(c => c.ticker == companyTicker).First(),
                            is_up = data[1] == "up",  // This turns up and down into true and false.
                        }).ToList();

            foreach (var prediction in picks)
            {
              if (includePrediction)
               {
                var p = new Prediction{
                  Member = prediction.Member,
                  Company = prediction.Company,
                  is_up = prediction.is_up
                 };
                db.AddToPredictions(p);
                }
            }

2) 或者，如果我不想保存预测，我可以分离预测。

foreach (var prediction in picks) {
                  if  (excludePrediction)
                  {
                      db.Detach(prediction)
                   }

}

Answer 1

原因在这里:

                      select new Prediction
                      {
                          Member = memberQuery.member,

这些行将(一旦 IEnumerable 被迭代；LINQ 是惰性的):

1. 实例化一个新的预测
2. 将该预测与现有成员相关联，*附加到db。

将实体的实例与附加实体相关联会自动将该实体添加到关联的附加实体的上下文中。

因此，一旦您开始迭代 predictionArray，上面的代码就会执行，您的上下文中就会有一个新实体。