public class Favorite
{
   [key]
   public int FavoriteId {get; set;}

   public int? BikeId {get; set;}
   public virtual Bike Bike {get;set;}

   public int? HelmetId {get;set;}
   public virtual Helmet Helmet {get;set;}

   public int? ShoeId {get;set;}
   public virtual Shoe Shoe{get;set;}

   public int UserId {get;set;}
   public virtual User User {get;set;}
}

public class Bike
{
   public int BikeId {get;set;}
   ...
   public virtual list<Favorties> Favorites {get;set;}
}

Favorites:
|   Id   |  BikeId  |  HelmetId  | ShoeId   | UserId        |
|   1    |    5     |            |          | snowburnt     |
|   2    |    6     |            |          | jonh          |
|   3    |          |     2      |          | snowburnt     |

Favorites:
|   ID    |   Differentiator   |   foreignKeyId    | UserId       |
|    1    |     Bike           |    5              | snowburnt    |
|    2    |     Bike           |    6              | john         |
|    3    |     Helmet         |    2              | snowburnt    |