How to load only IDs when use many-to-many relationship in Entity Framework?

I have the following models:

class Item
{
    public ICollection<string> GroupIds { get; set; }

    // Introduced for EF only
    public ICollection<Group> Groups { get; set; }
}

class Group
{
    public ICollection<string> ItemIds { get; set; }

    // Introduced for EF only
    public ICollection<Item> Items { get; set; }
}

class Store
{
    public ICollection<Item> Items { get; set; }

    public ICollection<Group> Groups { get; set; }
}

Store contains all Items and all Groups, but every Item may be related with one or multiple Group and every Group may be related with one or multiple Item.

How I can configure many-to-many relationship between Item and Group to load only GroupIds and ItemIds, but not objects itself?