Question

Suppose say I have Order, Items, OrderItems tables with Order and Items having n:n relationship and OrderItems being associative table. I have seen below two approaches for defining them.

1.Create Order and Items entities with “HasMany” relationship with OrderItems.

2.Create Order, Items and OrderItems entities with Order and Items having “ManytoMany” relationship and “OrderItems” containing Order and Item properties.

I have approach 1 which works fine but would like to know what approach 2 does.

Answer 1

If the relationship between Items and Orders is simple (merely that the relationship exists), then you would do a ManyToMany mapping between Items.Orders and Orders.Items. This would result in NHibernate generating and managing a simple cross reference table containing the two foreign keys.

On the other hand, if there is additional information that you need to record along with the two foreign keys, you must use a distinct entity or value object to capture that information, using HasMany on both sides.

Classes:

Order
  Id
  Name
  OrderItems

Item
  Id
  Name
  OrderItems

OrderItem
  Id
  Order
  Item
  Quantity

Mappings:

Order:

Id(c => c.Id);
Map(c => c.Name);
HasMany(c => c.OrderItems).KeyColumn("OrderId");

Item:

Id(c = c.Id);
Map(c => c.Name);
HasMany(c => c.OrderItems).KeyColumn("ItemId");

OrderItem:

Id(c => c.Id);
Map(c => c.Quantity);
References(c => c.Order);
References(c => c.Item);