Databases
StackOverflow2013
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PO
 

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    <p>This is a very big problem. What actually happens in your code is this:</p> <ul> <li>You load <code>Parent</code> from the database and get an attached entity</li> <li>You replace its child collection with new collection of detached children</li> <li>You save changes but during this operation all children are considered as <strong>added</strong> becasue EF didn't know about them till this time. So EF tries to set null to foreign key of old children and insert all new children => duplicate rows.</li> </ul> <p>Now the solution really depends on what you want to do and how would you like to do it? </p> <p>If you are using ASP.NET MVC you can try to use <a href="https://stackoverflow.com/questions/4653834/asp-net-mvc-ef4-poco-repository-how-to-update-relationships">UpdateModel or TryUpdateModel</a>.</p> <p>If you want just update existing children manually, you can simply do something like:</p> <pre><code>foreach (var child in modifiedParent.ChildItems) { context.Childs.Attach(child); context.Entry(child).State = EntityState.Modified; } context.SaveChanges(); </code></pre> <p>Attaching is actually not needed (setting the state to <code>Modified</code> will also attach the entity) but I like it because it makes the process more obvious.</p> <p>If you want to modify existing, delete existing and insert new childs you must do something like:</p> <pre><code>var parent = context.Parents.GetById(1); // Make sure that childs are loaded as well foreach(var child in modifiedParent.ChildItems) { var attachedChild = FindChild(parent, child.Id); if (attachedChild != null) { // Existing child - apply new values context.Entry(attachedChild).CurrentValues.SetValues(child); } else { // New child // Don't insert original object. It will attach whole detached graph parent.ChildItems.Add(child.Clone()); } } // Now you must delete all entities present in parent.ChildItems but missing // in modifiedParent.ChildItems // ToList should make copy of the collection because we can't modify collection // iterated by foreach foreach(var child in parent.ChildItems.ToList()) { var detachedChild = FindChild(modifiedParent, child.Id); if (detachedChild == null) { parent.ChildItems.Remove(child); context.Childs.Remove(child); } } context.SaveChanges(); </code></pre>
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    1. POThe relationship could not be changed because one or more of the foreign-key properties is non-nullable
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    1. USInteXX
    1. USLadislav Mrnka
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    1. COBut there is your interesting remark about using `.Clone()`. Do you have the case in mind that a `ChildItem` has other sub-child navigation properties? But in that case, wouldn't we want that the whole sub-graph is attached to the context since we would expect that all the sub-childs are new objects if the child itself is new? (Well, might be different from model to model, but let's assume the case that the sub-childs are "dependent" from the child like the childs are dependent from the parent.)
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    2. COIt would probably require "smart" clone.
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      1. USLadislav Mrnka
    3. COWhat if you don't want to have a Child's collection in your context? `http://stackoverflow.com/questions/20233994/do-i-need-to-create-a-dbset-for-every-table-so-that-i-can-persist-child-entitie`
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      1. USKirsten Greed
 

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