NHibernate and collection types

Question

I am trying to develop my very first project with Nhibernate 3.0. I've gone through loads of material (blogs, papers and samples) and I think I can understand the basics, pretty much.
I think I've understood the meaning of different types of collection but than when I see the examples found on the Internet I think I haven't really understood.
The documentation says that you should use a Set when you do not want duplicates and a List/Bag when you want to allow duplicates. Most of the samples I have found are based on a typical situation where you have Orders/OrderLines. If I have a look the mapping file of the Order I can see something like this:

<class name="OrderHeader"
        table="Orders">
    <id name="OrderId">
        <generator class="hilo"/>
    </id>
    <property name="OrderDate"/>
    <bag name="OrderItems" table="OrderDetails" cascade="all" inverse="true">
        <key column="OrderId"/>
        <one-to-many class="OrderDetail"/>
    </bag>
</class>

<class name="OrderDetail"
        table="OrderDetails">
    <id name="DetailId">
        <generator class="hilo"/>
    </id>
    <many-to-one name="ProductOrdered" column="ProductId"/>
    <many-to-one name="Order" column="OrderId" />
</class>

I had expcted to see the OrderItems mapped as a Set; an order will have unique OrderItems?
Am I right? At the same time I would expect to find a mapping for the class Product a bag of OrderItems

...  

<bag lazy="true" name="OrderItems">
  <key foreign-key="FK_OrderItems_Products">
    <column name="ProductCode" />
  </key>
  .....  
</bag>

...

In this situation a product would have a list of non-unique OrderItems.
Is there anything am I missing?
Forgive me all for the silly question :-s

score 2 · Answer 1 · answered Jan 12 '11 at 20:11

2

I would set it as a <list ...> as in my book two orderlines with the same characteristics are identical (.Equals should return true) - also, many consider the order(!) of orderlines to be important. :-)

answered Jan 12 '11 at 20:11

Goblin

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Could you explain that? You can have two orderlines with the same characteristic? I am missing something ... – LeftyX Jan 12 '11 at 22:06
1 pc. chair at 400$ == 1 pc. chair at 400$. For some reason many customers want to have the same type of product split between two orderlines to explain some relationship. – Goblin Jan 13 '11 at 17:31

score 1 · Accepted Answer · answered Jan 12 '11 at 19:29

1

The reason for this is that bag mapping is compatible with a .NET IList<T> mapping as Yads answered. Collections are typically mapped that way to avoid having to reference Iesi.Collections in your classes.

With NHibernate 2.1, you can use set mapping by declaring the collection as ICollection<T> and initializing it as a HashSet<T>. Using NHibernate 3 I believe you can declare the collection as ISet<T>.

I agree most real world collections should be mapped as sets.

answered Jan 12 '11 at 19:29

Jamie Ide

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Thanks Jamie, that's exactly what I did: I've declared a collection as ICollection and initialized as HashSet cause I didn't want to reference the Iesi.Collections. – LeftyX Jan 12 '11 at 22:10

score 0 · Answer 3 · answered Jan 12 '11 at 19:11

0

The reason most people implement it as a bag is because they can use the built in List<> class or IList<> interface to represent their collection.

answered Jan 12 '11 at 19:11

Vadim

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Yes, but the List allows you to have duplicates and in that situation (OrderLines) you should allow that. – LeftyX Jan 12 '11 at 22:04

NHibernate and collection types

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