Fluent Nhibernate Many to Many Mapping Way

I have two classes Order and Items

I want a method like this

class Order
{
    public virtual IList<Item> GetItems(Order order)
    {
         //get items for that order.
    }
}
class Item
{
    public virtual IList<Order> GetOrders(Item item)
    {
         //get all the orders in which that items is present.
    }
}

Is it write to create a method like this or instead should I create a property

     public virtual IList<Item> Items { get; set; }

And how should I do the mapping for this is nhibernate??

Apparently you have a many-to-many relationship: An order can have many items and an item can belong to many orders. In a relational database you need to express this with a separate table which I presume you have - let's assume this table is called OrdersItems.

Following the Store/Product example from the Fluent NHibernate documentation you would create an Items property in an Order and an Orders property in Item:

class Order
{
    public virtual IList<Item> Items { get; protected set; }
}

class Item
{
    public virtual IList<Order> Orders { get; protected set; }
}

And the mappings:

public class OrderMap : ClassMap<Order>
{
    public OrderMap()
    {
        HasManyToMany(x => x.Items)
           .Cascade.All()
           .Table("OrdersItems");
    }
}

public class ItemMap : ClassMap<Item>
{
    public ItemMap()
    {
        HasManyToMany(x => x.Orders)
           .Cascade.All()
           .Inverse()
           .Table("OrdersItems");
    }
}