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Here's my situation..
Suppose you have the following model entities, which represent single tables on their own:

Movies [ Movie_Id, Title, Rating, .. ]
Actors [ Actor_Id, FirstName, LastName, .. ]
Director [ Director_Id, FirstName, LastName, .. ]

And another entity/table called "Recommendations", which represents recommendations between users within a website. The idea is that a recommendation could be of any type, i.e. someone recommending an actor, or someone recommending a movie. Basically, the table should look something like this:

Recommendations [ Recommendation_Id, Object_Id, Object_Type, .. ]

And here's what i'm stuck into. How can I map these relationships in nHibernate with Fluent? I mean.. while mapping, I can't specify the Type (which table related to) 'cause that's determined on runtime, but I can't rely only on the Id 'cause by itself it cannot imply to which table belongs to.
For instance, imagine this record on Recommendations table:

Recommendation_Id--Object_Id--Object_Type
83001--4010123---"M"

Basically I'm storing a char identifier (in this case "M" stands for table "Movies") to know to which table Object_Id belongs to. I cannot just store Object_Id without Object_Type..

As final comments, I'd like to add that I've seen all the table-per-class, table-per-subclass, table-per-concrete-class examples, but I believe none of these fits in this situation, as Movies_Id, Actors_Id, Directors_Id, ... are all different between them, so is Recommendations_Id. I mean, there's no base class-child class inheritance here, they don't share Id's at all..

I hope I make myself clear. Thanks in advance.

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  • It should be noted that fluent nHibernate does not currently support many-to-any which is what I thought this would do. Unless I'm wrong, saving a recommendation when you update a movie won't happen seamlessly. Daniel's answer was closer to just about anything else I could find tho ;o) Commented Aug 1, 2013 at 20:47
  • If you want to map the other side of the any relationship, as in Movie.Recommendations, I believe you could do this with a one-to-many and a where="Object_Type='M'" attribute on the collection, which you can do with Fluent NHibernate. Commented Aug 28, 2013 at 2:20

1 Answer 1

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14

The NHibernate mapping you are looking for is <any/>. Here are a few resources to help you get up to speed with this NHibernate mapping feature:

I believe the *.hbm.xml that you are shooting for is something like this:

<class name="Recommendation" table="Recommendations">
  <id name="Id">
    <column name="Recommendation_Id" />
    <generator class="native"/>
  </id>

  <any name="RecommendedObject" id-type="System.Int32" meta-type="System.String">
    <meta-value value="M" class="Movie"/>
    <meta-value value="A" class="Actor"/>
    <meta-value value="D" class="Director"/>
    <column name="Object_Type"/>
    <column name="Object_Id"/>
  </any>

  <!-- other stuff ... -->
</class>

You should be able to accomplish that with Fluent NHibernate like so in Recommendation's mapping:

ReferencesAny(x => x.RecommendedObject)
    .IdentityType<int>()
    .EntityTypeColumn("Object_Type")
    .EntityIdentifierColumn("Object_Id")
    .AddMetaValue<Movie>("M")
    .AddMetaValue<Actor>("A")
    .AddMetaValue<Director>("D");
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3 Comments

THANKS A LOT !!!! I've tested it and seems to work fine. Seriously, a million thanks Daniel! :=)
Could you please show an example how to query over the object RecommendedObject with filtering by Object_Id ?
@Christian, are you referring to stackoverflow.com/questions/21940396/…?

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