title: Entity Framework 6: Multicolumn unique index featuring navigation property
body: <p>How can I set multicolumn index on model like that:</p>
  
  <pre><code>public class Meta
  {
      [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
      [Key]
      public Guid Id { get; set; }
  
      [Index("MetaPeriodDateUnq", IsUnique = true, Order = 2)]
      [Required] 
      public DateTime Date { get; set; }
  
      [Index("MetaPeriodDateUnq", IsUnique = true, Order = 1)]
      [Required] 
      public virtual PeriodType Period { get; set; }
  
      /*
      ...
      */
  }
  
  public class PeriodType
  {
      [Key]
      public Guid Id { get; set; }
  
      /*
      ...
      */
  }
  </code></pre>
  
  <p>After DB initialization there is only "MetaPeriodDateUnq" index mentioning Meta.Date column, but I'm relying on Meta.Date + Meta.Period.Id uniqueness.</p>