防止Entity Framework重复插入关联对象
Entity Framework在數(shù)據(jù)庫(kù)與對(duì)象映射上做了很多工作，除了將數(shù)據(jù)庫(kù)里的表映射成相應(yīng)的對(duì)象以外，它還能夠自動(dòng)處理表之間的外鍵關(guān)系，并且可以用導(dǎo)航屬性（Navigation Property）的方式在對(duì)象層面上表示這些關(guān)系。
一般來說，當(dāng)你插入一個(gè)對(duì)象時(shí)，Entity Framework默認(rèn)會(huì)自動(dòng)將對(duì)象通過導(dǎo)航屬性關(guān)聯(lián)的對(duì)象也插入到數(shù)據(jù)庫(kù)里面去，大部分情況下，這是我們想要的結(jié)果。當(dāng)然，如果關(guān)聯(lián)的對(duì)象已經(jīng)存在于數(shù)據(jù)庫(kù)當(dāng)中時(shí)，Entity Framework會(huì)避免重復(fù)插入對(duì)象。但問題是，這個(gè)檢查對(duì)象已經(jīng)存在避免重復(fù)插入數(shù)據(jù)的功能，好像只在一個(gè)Context（環(huán)境）下有效，即下面的代碼是可以正常執(zhí)行的：
??????????? using (var context = new TestContext())
??????????? {
??????????????? var milestone = new Milestone()
??????????????? {
??????????????????? Title = "測(cè)試?yán)锍瘫?/span>",
??????????????????? StartDate = DateTime.Now,
??????????????????? DueDate = DateTime.Now + TimeSpan.FromDays(30)
??????????????? };
?
??????????????? context.Milestones.Add(milestone);
??????????????? context.SaveChanges();
?
??????????????? id = milestone.Id;
?
??????????????? var project = new Project()
??????????????? {
????????????????? ??Title = "測(cè)試項(xiàng)目",
??????????????????? StartDate = DateTime.Now,
??????????????????? DueDate = DateTime.Now + TimeSpan.FromDays(30),
??????????????????? Owner = "測(cè)試用戶"
??????????????? };
?
??????????????? project.Children.Add(milestone);
??????????????? context.Project.Add(project);
??????????????? context.SaveChanges();
??????????? }
?
而如果對(duì)象是跨Context（環(huán)境）的話，或者基于現(xiàn)有對(duì)象復(fù)制的對(duì)象（包括主鍵也復(fù)制的情況），這就會(huì)產(chǎn)生重復(fù)插入的問題，因?yàn)樾聫?fù)制的對(duì)象，Entity Framework沒有辦法跟蹤對(duì)象的狀態(tài)，“誤以為”對(duì)象是一個(gè)全新的對(duì)象，比如，下面這段代碼就會(huì)導(dǎo)致Entity Framework拋出一個(gè)異常，異常根據(jù)Entity對(duì)象的數(shù)據(jù)庫(kù)約束不同，可能會(huì)報(bào)告不同的錯(cuò)誤信息—這個(gè)問題一開始讓我迷惑了好幾天：
??????? public static void Main(string[] args)
??????? {
??????????? int id = 0;
??????????? using (var context = new TestContext())
??????????? {
??????????????? var milestone = new Milestone()
??????????????? {
??????????????????? Title = "測(cè)試?yán)锍瘫?/span>",
??????????????????? StartDate = DateTime.Now,
??????????????????? DueDate = DateTime.Now + TimeSpan.FromDays(30)
??????????????? };
?
??????????????? context.Milestones.Add(milestone);
??????????????? context.SaveChanges();
?
??????????????? id = milestone.Id;
??????????? }
?
??????????? using ( var context = new TestContext())
??????????? {
??????????????? var project = new Project()
??????????????? {
??????????????????? Title = "測(cè)試項(xiàng)目",
??????????????????? StartDate = DateTime.Now,
??????????????????? DueDate = DateTime.Now + TimeSpan.FromDays(30),
??????????????????? Owner = "測(cè)試用戶"
??????????????? };
??????????????? var child = new Milestone() {
????? ??????????????Id = id
??????????????? };
?
??????????????? project.Children.Add(child);
???????????????????????????????
??????????????? context.Project.Add(project);
??????????????? context.SaveChanges();
??????????? }
??????? }
?
執(zhí)行上面這段代碼，Entity Framework會(huì)在最后一個(gè)context.SaveChanges()上面拋出DbUpdateException，詳細(xì)信息是：“{"The conversion of a datetime2 data type to a datetime data type resulted in an out-of-range value.\r\nThe statement has been terminated."}”。這個(gè)異常一開始看上去太怪異了， 明明我將要保存的Project對(duì)象的所有DateTime類型都已經(jīng)賦值（而且賦值都在范圍內(nèi)）了，為什么還說超出賦值范圍呢？
后面才發(fā)現(xiàn)，這是因?yàn)?#xff0c;Entity Framework在插入project對(duì)象是，看到它的關(guān)聯(lián)對(duì)象列表Children里，有一個(gè)Milestone對(duì)象，而Milestone對(duì)象是重新復(fù)制的（只復(fù)制了ID）—這個(gè)場(chǎng)景是因?yàn)橛脩粼诰W(wǎng)頁(yè)上創(chuàng)建一個(gè)項(xiàng)目時(shí)，可以從里程碑列表里選擇一個(gè)事先創(chuàng)建好了的里程碑。由于Entity Framework沒有辦法跟蹤這個(gè)新復(fù)制的Milestone對(duì)象的狀態(tài)，所以它“誤認(rèn)為”這個(gè)對(duì)象是一個(gè)新的對(duì)象，因此重新插入這個(gè)對(duì)象，而這個(gè)對(duì)象又沒有設(shè)置一些必要的日期屬性，導(dǎo)致了前面那個(gè)異常。
既然搞明白了道理，修復(fù)起來也很簡(jiǎn)單，就是顯式告訴Entity Framework跟蹤這個(gè)對(duì)象—通過把第二個(gè)using段改成下面這樣：
??????????? using ( var context = new TestContext())
??????????? {
??????????????? var project = new Project()
??????????????? {
??????????????????? Title = "測(cè)試項(xiàng)目",
??????????????????? StartDate = DateTime.Now,
?????????????????? ?DueDate = DateTime.Now + TimeSpan.FromDays(30),
??????????????????? Owner = "測(cè)試用戶"
??????????????? };
??????????????? var child = new Milestone() {
??????????????????? Id = id
??????????????? };
?
??????????????? project.Children.Add(child);
????????????? ??var adapter = context as IObjectContextAdapter;
??????????????? adapter.ObjectContext.AttachTo("Milestones", child);
???????????????????????????????
??????????????? context.Project.Add(project);
??????????????? context.SaveChanges();
??????????? }
?
注意：我用的是Entity Framework CTP 5，采用的是代碼優(yōu)先（code first）的方式創(chuàng)建的數(shù)據(jù)庫(kù)，但是本文提到的問題在數(shù)據(jù)庫(kù)優(yōu)先和模型優(yōu)先的情況里都是一樣的。
因?yàn)樵诰W(wǎng)上找了好多文章都沒有提到這個(gè)問題，所以在這里記錄下來。
重現(xiàn)代碼：/Files/killmyday/codefirstef.zip
轉(zhuǎn)載于:https://www.cnblogs.com/killmyday/archive/2010/12/17/1909630.html
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