Many-to-many collection mapping in NHibernate

Question

My application is a multi-user podcast aggregator, using NHibernate 2.1 (and .NET 4.0 if that changes anything). I'm also a complete NHibernate n00b.

Details of each podcast are only stored once, and Users subscribe to podcasts, so there is a many-to-many mapping between podcasts and users which I have mapped in my database with a Subscriptions table:

Subscriptions(UserId, FeedId)

I also have a UserFeedItems table which stores per-user-per-item information:

UserFeedItems(UserId, FeedItemId, IsNew, ListenCount, etc.)

My object model is a bit different:

class Podcast {
    IList<PodcastFeedItem> FeedItems { get; set; }
    bool HasNew {
        get {
            // return true if any of the FeedItems are new
        }
    }
}

class PodcastFeedItem {
    bool IsNew { get; set; }
}

class User {
    IList<PodcastFeed> Subscriptions { get; set; }
}

What mappings do I need in NHibernate to correctly map the relational model to the object model? Have I gone to far by defining those "link" tables listed above? Ordering may be important here, as obviously I need to keep the feeds organised in descending chronological order.

I've read through the documentation on collection mapping, but I'm struggling to fit the examples to my own scenario.

Answer 1

You don't need the lookup table. The answer to this question provides the details on how to perform a many to many in NHibernate

Answer 2

Its a classic Many-to-Many where a Podcast can have many subscribers and a subscriber can subscribe to many podcasts. You map it like this:

<class name="App.Core.Domain.User, App.Core" table="users">
    <set name="SubscribedPodcasts" table="subscriptions" inverse="false" cascade="all">
      <key column="userid"/>
      <many-to-many class="App.Core.Domain.Podcasts, App.Core" column="podcastid"/>
    </set>
</class>


<class name="App.Core.Domain.Podcast, App.Core" table="podcasts">
    <set name="SubscribedUsers" table="subscriptions" inverse="false" cascade="all">
      <key column="podcastid"/>
      <many-to-many class="App.Core.Domain.User, App.Core" column="userid"/>
    </set>
</class>

If you really want to store the index in the database rather than having NH order the results (I have never had to do this as its better to order by a column and have NH provide your indexes). then add

<index-many-to-many
        column="column_name"                
        class="ClassName"                   
/>

To the mapping