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PONew object, but error: an object with the same key already exists in the objectstatemanager. the objectstatemanager cannot track multiple (...)
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copied!<p><code>Student</code> and <code>Teacher</code> are i relationship <code>many-to-many</code>.</p> <p>When I have <code>Student</code> existed in database, and want to add new <code>Teacher</code>, I try code below.</p> <p>But in line: <code>addedTeachers.ForEach(a => dbStudent.Teacher.Add(a));</code> I get error </p> <p>"<code>an object with the same key already exists in the objectstatemanager. the objectstatemanager cannot track multiple objects with the same key</code>".</p> <p>What's wrong?</p> <pre><code>void Update(Student s) { using(var context = new MyEntities(connectionString)) { context.ContextOptions.LazyLoadingEnabled = false; var dbStudent = context.Student.Include("Teacher").Where(a => a.Id == s.Id).SingleOrDefault(); var dbTeachers = dbStudent.Teacher.ToList(); var newTeachers = s.Teacher.ToList(); var addedTeachers = newTeachers.Except(dbTeachers).ToList(); var deletedTeachers = dbTeachers.Except(newTeachers).ToList(); addedTeachers.ForEach(a => dbStudent.Teacher.Add(a)); deletedTeachers.ForEach(a => dbStudent.Teacher.Remove(a)); context.SaveChanges(); } } </code></pre> <p><strong>EDIT</strong></p> <p>What's strange else:</p> <p>just before this line with exception <code>dbStudent.Teacher.Count</code> is 0. But after exception is 1. Of course <code>addedTeachers.Count</code> is also 1, but debugger doesn't reach the next line.</p>

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