c# - Trying to get arround "Multiplicity constraint violated." / SaveChanges is Changing Values -

i'm trying overcome "multiplicity constraint violated." error. have created simple / contrived example demonstrate issue. in example, have task, has collection of sub-tasks , task can sub-task of 1 or more tasks. want able order sub-tasks. i'm open other suggestions on how have many-to-many relationship keeps track of order.

there 3 test bellow have different problems. 1 find interesting third test orderedtask created , values correct until value taskid changed someplace inside of 'context.savechanges()'

solution on git: https://github.com/jrswenson/orderedmanytomany

public class task {     private icollection<orderedtask> subtasks;      public task()     {         subtasks = new list<orderedtask>();     }      public int id { get; set; }     public string description { get; set; }      public virtual user assigneduser { get; set; }     public int? assigneduserid { get; set; }      [inverseproperty("parent")]      public virtual icollection<orderedtask> subtasks     {         { return subtasks; }         set { subtasks = value; }     } }  public class orderedtask {     public virtual task parent { get; set; }      [key, column(order = 1), foreignkey("parent")]     public int parentid { get; set; }      public virtual task task { get; set; }      [key, column(order = 2), foreignkey("task")]     public int taskid { get; set; }      public int order { get; set; } } 

in unit test:

[testclass] public class tasktest {     [testmethod]     public void createtasks()     {         var context = new context();          var task = context.tasks.firstordefault(i => i.description == "lev1") ?? new task { description = "lev1" };         if (context.tasks.any(i => i.id == task.id) == false)             context.tasks.add(task);          var sub1 = context.tasks.firstordefault(i => i.description == "lev2-1") ?? new task { description = "lev2-1" };         if (context.tasks.any(i => i.id == sub1.id) == false)             context.tasks.add(sub1);          context.savechanges();     }      //this throw "multiplicity constraint violated" error     [testmethod]     public void insertsubtasks()     {         var context = new context();          var task = context.tasks.firstordefault(i => i.description == "lev1");         assert.isnotnull(task);          var sub1 = context.tasks.firstordefault(i => i.description == "lev2-1");         assert.isnotnull(sub1);          if (task.subtasks.any(i => i.taskid == sub1.id) == false)         {             var ot = new orderedtask { parent = task, task = sub1, order = task.subtasks.count + 1 };                            task.subtasks.add(ot);         }          context.savechanges();     }      //this doesn't throw exception.     //the orderedtask added database ,     //the table has correct values.     //unfortunately, if ran second time,      //task.subtasks empty , causes duplicate key error.     [testmethod]     public void insertsubtasks2()     {         var context = new context();          var task = context.tasks.firstordefault(i => i.description == "lev1");         assert.isnotnull(task);          var sub1 = context.tasks.firstordefault(i => i.description == "lev2-1");         assert.isnotnull(sub1);          if (task.subtasks.any(i => i.taskid == sub1.id) == false)         {             var ot = new orderedtask { parent = task, task = sub1, order = task.subtasks.count + 1 };             context.orderedtasks.add(ot);         }          context.savechanges();     }      //this doesn't throw exception first time, on      //second time.     //the orderedtask created , has correct values after      //added task.subtasks, somewhere in context.savechanges value     //for parentid , taskid both set same value of parentid.      //second time test ran task.subtasks has value (unlike test above)     //, values not correct.     [testmethod]     public void insertsubtasks3()     {         var context = new context();          var task = context.tasks.firstordefault(i => i.description == "lev1");         assert.isnotnull(task);          var sub1 = context.tasks.firstordefault(i => i.description == "lev2-1");         assert.isnotnull(sub1);          if (task.subtasks.any(i => i.taskid == sub1.id) == false)         {             var ot = new orderedtask { parent = task, task = sub1, order = task.subtasks.count + 1 };             context.orderedtasks.add(ot);             task.subtasks.add(ot);         }          context.savechanges();     } }