title: How to bind to data thats not in my model using MVVM
body: <p>I currently have a two tables.</p>
  
  <pre><code>Project
      int Number
      string Name
      List&lt;WorkOrder&gt; WorkOrders
  
  WorkOrder
      int Number
      DateTime Date
      Project Project
  </code></pre>
  
  <p>I have a WPF window with a Master/Detail view using a ListView showing Projects and a DataGrid showing the WorkOrders wired up using MVVM and a single viewmodel. Everything works great but my coworkers want to make some changes. The first digit in the WorkOrder number corresponds to the type of material, i.e. 1=Wood, 2=Steel, 3=Plastic. They want the Project list to include the material categories to filter down the detail view.</p>
  
  <pre><code>Projects
   - 8888 - Some Project
     - Wood
     - Metal
     - Plastic
   - 9999 - Another Project
     - Wood
     - Metal
     - Plastic
  </code></pre>
  
  <p>Im just not sure how I databind to something that isn't in my data.  I created a TreeView and filled the material nodes using a Converter in my HierarchicalDataTemplate but there is nothing to bind to in my ViewModel to see what is selected from inside the ViewModel.  I hope this makes sense. Thanks in advance for our reply.</p>