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Asked: 2026-05-11T00:32:58+00:00

I’m not sure how to ask the question, for I don’t know what I

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I’m not sure how to ask the question, for I don’t know what I don’t know, and therefore I don’t know the proper terminology for what I’m trying to get the answer to. I will explain my scenario, in hopes that it will help:

I’ve got three tables, a Book table, a Tag table and a BookTag lookup table.

Each book has an ID, a Title (for starters) Each tag has an ID, and a Title Each BookTag has an ID, a BookID, and a TagID.

A book can be tagged with multiple tags, and a tag can be used on more than one BookID.

I’ve got my objects setup in this fashion:

Book.cs int BookID string Title List<BookTag> Tags  Tag.cs int TagID string Title  BookTag.cs int ID int BookID int TagID

I would like the Books.cs class to have a collection of Tags, and not BookTags, but I cannot seem to get the mapping right in NHibernate. This is what I’ve got for the Book.hbm.xml file:

<?xml version='1.0' encoding='utf-8' ?> <hibernate-mapping xmlns='urn:nhibernate-mapping-2.2' assembly='DomainModel' namespace='DomainModel.Books'>   <class name='DomainModel.Books.Book' table='Books'>     <id name='BookID' type='Int32' unsaved-value='0'>       <generator class='native'/>     </id>     <property name='Title' type='String' not-null='true'/>     <set lazy='true' name='Tags' table='BookTags' generic='true' inverse='true' cascade='delete'>       <key column='BookID'/>       <one-to-many class='DomainModel.Books.BookTag, DomainModel'/>     </set>   </class> </hibernate-mapping>

And this is my BookTag.hbm.xml:

<?xml version='1.0' encoding='utf-8' ?> <hibernate-mapping xmlns='urn:nhibernate-mapping-2.2' assembly='DomainModel' namespace='DomainModel.Books'>   <class name='DomainModel.Books.BookTag' table='BookTags'>     <id column='BookTagID' name='BookTagID' type='Int32' unsaved-value='0'>       <generator class='native'/>     </id>     <many-to-one name='Tag'>       <column not-null='true' name='TagID'/>     </many-to-one>     <many-to-one name='Book'>       <column not-null='true' name='BookID'/>     </many-to-one>   </class> </hibernate-mapping>

Under this model, I can get to the tag I want by using my object model: Book.Tags[0].Tag, but that just seems inefficient. Can I use NHibernate to map out the BookTags.TagID with the Tags.TagID in the database so that I can get Book.Tags[0] to return a Tag object, instead of a BookTags object? I didn’t know of a better way to associate Books to tags so that a tag used on Book1 can be used on Book2 without adding a new entry to the Tags table.

I hope this makes at least some sense. Let me know if you need further clarification. I’ll post my solution here if I figure it out before someone answers.

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1 Answer

  1. You don’t need a BookTag class at all. You can map Book.Tags collection as many-to-many. To do this you will specify BookTag in the map to connect the association. Look here in section 6.8 Bidirectional Associations.

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