title: Exclude DbContext from migrations to avoid Context-Parameter
body: <p>I'm using ASP.NET EF Core with MySQL (Pomelo.EntityFrameworkCore.MySql driver). Cause such a context need some lines of specific configuration which the default DbContext with MSSQL doesn't have, I created a MySqlContext:</p>
  
  <pre><code>public class MySqlContext : DbContext {
      string connectionString;
      // ...
  }
  </code></pre>
  
  <p>So all my DbContexts are inherited from those class:</p>
  
  <pre><code>public class MyDbContext: MySqlContext {
      public DbSet&lt;MyModel&gt; MyModels{ get; set; }
      // ...
  }
  </code></pre>
  
  <p>In my project I have currently one context (DbContext) which contains DbSets. And the MySqlContext, which isn't a real context like this because it has no DbSets and only acts as a wrapper for the MySQL configuration so that I can better re-use them. </p>
  
  <p>But it seems that the migrations will see them as two different contexts because when I add a migration using command <code>Add-Migration Test</code> I got the error </p>
  
  <blockquote>
    <p>More than one DbContext was found. Specify which one to use. Use the '-Context' parameter for PowerShell commands and the '--context' parameter for dotnet commands.</p>
  </blockquote>
  
  <p>So on every migration I have to add <code>-Context MyContext</code> to the command. I would like to avoid this, because as I said the second MySqlContext is not a real context which holds models and migrations. Is there a way to tell the migration tool this? I need something like the [NotMapped] attribute for EF models, which tells EF: Dont store this property to the database. Like this I need to tell EF: Ignore the MySqlContext class cause they need no migrations. </p>