保存父项后尝试包含一对多关系项时发生NullReferenceException [英] NullReferenceException while trying to including a one-to-many relationship item after saving parent

问题描述

框架::我正在使用MVC 3 + EntityFramework 4.1代码优先.

Framework: I'm using using MVC 3 + EntityFramework 4.1 Code-First.

概念:一个立法实体有许多Provision实体.这个想法是，用户输入一个立法实体，将其保存，然后将其保存的函数将其传递给另一个函数，以查看该立法是否具有ShortTitle.如果是这样，则将其格式化为措词正确的字符串，并将其包括为立法的第一条规定，然后将更改保存到db.

Concept: One Legislation entity has many Provision entities. The idea is that the user enters a Legislation entity, that gets saved then the function that saves it passes it along to another function to see whether that Legislation has a ShortTitle. If it does, then it formats it into a properly worded string and includes it as the Legislation's first Provision, then saves the changes to db.

问题:问题是，我尝试用不同的方式对其进行编码，我不断收到NullReferenceException，并告诉我使用"new"关键字创建一个新的对象实例，并指向我到第二个功能中的savedLegislation.Provisions.Add(provision);行.

Issue: The problem is, I've tried coding it in different ways, I keep getting a NullReferenceException, telling me to create a new object instance with the "new" keyword, and points me to the savedLegislation.Provisions.Add(provision); line in my second function.

以下是有争议的两个功能，第一个功能保留了适当的立法:

Here are the two functions at issue, this first one saves the Legislation proper:

public Legislation Save(NewLegislationView legislation)
{
    Legislation newLegislation = new Legislation();
    // Simple transfers
    newLegislation.ShortTile = legislation.ShortTile;
    newLegislation.LongTitle = legislation.LongTitle;
    newLegislation.BillType = legislation.BillType;
    newLegislation.OriginatingChamber = legislation.OriginatingChamber;
    newLegislation.Preamble = legislation.Preamble;

    // More complicated properties
    newLegislation.Stage = 1;
    this.NumberBill(newLegislation); // Provides bill number
    newLegislation.Parliament = db.LegislativeSessions.First(p => p.Ending >= DateTime.Today);
    newLegislation.Sponsor = db.Members.Single(m => m.Username == HttpContext.Current.User.Identity.Name);

    // And save
    db.Legislations.Add(newLegislation);
    db.SaveChanges();

    // Check for Short titles
    this.IncludeShortTitle(newLegislation);

    // return the saved legislation
    return newLegislation;
}

第一个调用的第二个函数负责检查ShortTitle是否不为空，并创建与该立法相关的Provision，然后保存更改.

And the second function which is invoked by the first one deals with checking whether ShortTitle is not empty and create a Provision that is related to that Legislation, then save changes.

public void IncludeShortTitle(Legislation legislation)
{
    var savedLegislation = db.Legislations.Find(legislation.LegislationID);
    if (savedLegislation.ShortTile.Any() && savedLegislation.ShortTile.ToString().Length >= 5)
    {
        string shortTitle = "This Act may be cited as the <i>" + savedLegislation.ShortTile.ToString() + "</i>.";
        var provision = new Provision()
        {
            Article = Numbers.CountOrNull(savedLegislation.Provisions) + 1,
            Proponent = savedLegislation.Sponsor,
            Text = shortTitle
        };
        savedLegislation.Provisions.Add(provision);
        db.SaveChanges();
    }
}

我一直在研究SaveChanges()的工作方式，以及它是否正确地返回更新后的实体，它确实可以(因为在第二个函数中查找它没有问题).如果它能正常工作，并且在第二个功能中找到了立法并重新创建了该规定，那么我看不出它不断散发出的空"引用是什么.

I've been researching how SaveChanges() works and whether it is properly returning the updated entity, it does (since I get no issue looking it up in the second function). If it works properly, and the legislation is found and the provision is newly created in the second function, I don't see what is the "null" reference it keeps spitting out.

推荐答案

在这种情况下，空引用将为savedLegislation.Provisions.当EF从db.Legislations.Find(...)方法返回您的Legislation实例时，Provisions集合将不会初始化为新的List<Provision>.

The null reference in this case would be savedLegislation.Provisions. The Provisions collection won't be initialized to a new List<Provision> when EF returns your Legislation instance from the db.Legislations.Find(...) method.

我要尝试的第一件事是这样的:

The first thing I'd try is something like this:

var savedLegislation = db.Legislations
    .Include("Provisions")
    .First(l => l.LegislationID == legislation.LegislationID);

...但是我还考虑仅使用传递到方法中的legislation实例，而不是再次从数据库中获取它.

... but I'd also consider just using the legislation instance that was passed into the method rather than fetching it from the database again.

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