System.Data.Entity not referenced when using Entity Framework

I started getting a compilation error telling with a message telling me "CS0012: The type 'System.Data.Objects.DataClasses.EntityObject' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'". This really confused me since the solution built fine but the error only occurred at run time.

Obviously, it is all about adding a reference to System.Data.Entity to my Web project right? I checked and it is already there. Removed it re-added it. Checked for the correct version but still it doesn't like it! Why?

This came about when I used an existing Models project from a different solution in a brand new MVC 2 web application. So I checked the differences between the working solution and the new solution. Both had the same reference in the Web project so what's wrong?

Solution

Finally, I found that I had to add the System.Data.Entity assembly in the web.config too! See line 6 below:

<compilation debug="true" targetFramework="4.0">
  <assemblies>
    <add assembly="System.Web.Abstractions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
    <add assembly="System.Web.Routing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
    <add assembly="System.Web.Mvc, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
    <add assembly="System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
  </assemblies>
</compilation>

This miraculously made my page start working as expected again.