You'll need to fix the design otherwise you'll be pulling your hair out when it comes time to query the data. Assignable items should exist in a separate table maybe named Accessory including a primary key that identifies a Mouse (1) and a Keyboard (2). Create a third table that relates an Accessory to a Product named ProductAccessories. The ProductAccessories table will have a ProductId and an AccessoryId column which makeup the primary key for the ProductAccessories table.
The Laptop in your example will have two records in the ProductAccessories table.
1 (laptop) | 1 (mouse)
1 (laptop) | 2 (keyboard)
By the way, this is called a many-to-many relationship and explained in another one of your threads on this subject. Please see the Entity Framework docs to learn navigation basics.
Relationships, navigation properties, and foreign keys
Code First Data Annotations
Fluent API - Relationships