title: Adding & Removing Associations - Entity Framework
body: <p>I'm trying to get to grips with EF this week and I'm going ok so far but I've just hit my first major snag. I have a table of items and a table of categories. Each item can be 'tagged' with many categories so I created a link table. Two columns, one the primary ID of the item, the other the primary ID of the category. I added some data manually to the DB and I can query it all fine through EF in my code. </p>
  
  <p>Now I want to 'tag' a new item with one of the existing categories. I have the category ID to add and the ID of the Item. I load both as entities using linq and then try the following.</p>
  
  <pre><code>      int categoryToAddId = Convert.ToInt32(ddlCategoriesRemaining.SelectedValue);
    var categoryToAdd = db.CollectionCategorySet.First(x =&gt; x.ID == categoryToAddId);
    currentCollectionItem.Categories.Add(categoryToAdd);
    db.SaveChanges(); 
  </code></pre>
  
  <p>But I get "Unable to update the EntitySet 'collectionItemCategories' because it has a DefiningQuery and no  element exists in the  element to support the current operation."</p>
  
  <p>Have I missed something? Is this not the right way to do it? I try the same thing for removing and no luck there either.</p>