This exception message doesn’t mean it requires a primary key to be defined in your database, it means it requires a primary key to be defined in your class.
Although you’ve attempted to do so:
private Guid _id; [Key] public Guid ID { get { return _id; } }
This has no effect, as Entity Framework ignores read-only properties. It has to: when it retrieves a Fruits record from the database, it constructs a Fruit object, and then calls the property setters for each mapped property. That’s never going to work for read-only properties.
You need Entity Framework to be able to set the value of ID. This means the property needs to have a setter.