title: Store computed property with Entity Framework Core
body: <p>I'll try and illustrate my question with an oversimplified example:
  Imagine I have a domain entity like this:</p>
  
  <pre><code>public class Box
  {
      public int Id { get; set; }
      public int Height { get; set; }
      public int Width { get; set; }
      public int Depth { get; set; }
      public int Volume =&gt; Height * Width * Depth;
  }
  </code></pre>
  
  <p>Where I'm doing a calculation (volume) based on every other property.
  Now say I want to store this class using Entity Framework Core. Is there any way I can make EF core store the current value of Volume in its own column when i persist the entity?</p>
  
  <p>That's my core question. I'm not allowed to share my actual code, but here's some more in-depth information about my real-world entity(let's call it "Box"):</p>
  
  <ul>
  <li>My Box has 18 properties of which 6 are collections of sub entities, defining one-to-many relationships with these, so you could call it an aggregate root. </li>
  <li>Every property, including the sub-entity collections contribute to a "grand total", let's call it "volume", that's exposed as a public property. Whenever a property is changed or a sub-entity is added or removed, the volume will change.</li>
  <li>In my view I need to list out every Box, but I only need to show 5 properties, one of them being the volume, which is why I'd like to store it in a database field every time i persist an entity.</li>
  </ul>
  
  <p>Some ideas on how to solve this:</p>
  
  <ol>
  <li>Completely hydrate every Box, and calculate the volume for each and THEN project this to a summary. This includes joining all seven tables for each box with sub-entities, doing the calculation for each box and projecting to simplified view models. To me this just seems like a lot of overhead to get a number i knew at the time of persisting.</li>
  <li>Create a "persistence DTO" for Box and all sub-entities and then just map the result of Volume to a volume auto-property on the dto when storing. This also seems like a lot of overhead for just storing one number, and It also seems to be in complete discord with how EF is supposed to work. I just want to persist my entities.</li>
  <li>I could go proper OO on Box and create private fields and proper setters for every property, that updates a private volume field whenever a setter is called. This would also include writing methods on Box for manipulating all collections of sub-entities, and presenting these as read-only collections. This would lead to a lot of overhead private fields and code duplication on every setter, but does seem more "prudent" than the above alternatives.</li>
  <li>I could turn Volume in to a CalculateVolume() method and create a Volume-property using fluent API, and then populate that property in a SaveChanges() override on the context. But overriding SaveChanges is the kind of EF gung-ho I'm not comfortable doing.</li>
  <li>I Could to something like this:</li>
  </ol>
  
  <pre><code>public class Box
  {
      public int Id { get; set; }
      public int Height { get; set; }
      public int Width { get; set; }
      public int Depth { get; set; }
      public int Volume {
          get =&gt; CalculateVolume(); 
          private set =&gt; _volume = value; }
      private int _volume;
      private int CalculateVolume() =&gt; Height * Width * Depth;
  }
  </code></pre>
  
  <p>Which does seem to do what I want, but for some reason feels like cheating, and polluting my domain entity. Also I'm unsure this actually works in all cases, but this is my preferred solution at the time of writing.</p>
  
  <p>I'd prefer to be able to just configure this using fluent API. I noticed the PropertyBuilder.ValueGeneratedOnAdd()-method description says "The value may be generated by a client-side value generator or may be generated by the database as part of saving the entity.", but I can't find any examples of client-side value-generation.</p>
  
  <p>Any and all reasonable feedback welcome.</p>
  
  <p>EDIT:
  Just to clarify: The actual calculation is pretty complex and uses values from 7 different tables. There's also a weighting of each property involved. The Box example at the start is over simplified and for explanation purposes only. Suffice to say, I need to keep the calculation in my code. I just want to store the result.</p>