Question

I’m currently using NHibernate as my data access layer, using Fluent NHibernate to create the mapping files for me. I have two classes, TripItem and TripItemAttributeValue, which have a many-to-many relation between them.

The mapping is as follows:

public class TripItemMap : ClassMap<TripItem2> {     public TripItemMap()     {         WithTable('TripItemsInt');         NotLazyLoaded();          Id(x => x.ID).GeneratedBy.Identity().WithUnsavedValue(0);         Map(x => x.CreateDate, 'CreatedOn').CanNotBeNull();         Map(x => x.ModifyDate, 'LastModified').CanNotBeNull();          /* snip */          HasManyToMany<TripItemAttributeValue>(x => x.Attributes).AsBag()             .WithTableName('TripItems_TripItemAttributeValues_Link')             .WithParentKeyColumn('TripItemId')             .WithChildKeyColumn('TripItemAttributeValueId')             .LazyLoad();     } }  public class TripItemAttributeValueMap : ClassMap<TripItemAttributeValue> {     public TripItemAttributeValueMap()     {         WithTable('TripItemAttributeValues');          Id(x => x.Id).GeneratedBy.Identity();         Map(x => x.Name).CanNotBeNull();          HasManyToMany<TripItem2>(x => x.TripItems).AsBag()             .WithTableName('TripItems_TripItemAttributeValues_Link')             .WithParentKeyColumn('TripItemAttributeValueId')             .WithChildKeyColumn('TripItemId')             .LazyLoad();     } } 

At some point in my application I fetch existing attributes from the database, add them to tripItem.Attributes, then save the tripItem object. In the end, the TripItems_TripItemAttributeValues_Link never gets any new records, resulting in the relations not being persisted.

If it helps, these are the mapping files generated by Fluent NHibernate for these classes:

<?xml version='1.0' encoding='utf-8'?> <hibernate-mapping xmlns='urn:nhibernate-mapping-2.2' default-lazy='true' assembly='ETP.Core' namespace='ETP.Core.Domain'>   <class name='TripItem2' table='TripItemsInt' xmlns='urn:nhibernate-mapping-2.2' lazy='false'>     <id name='ID' column='ID' type='Int32' unsaved-value='0'>       <generator class='identity' />     </id>     <property name='CreateDate' column='CreatedOn' type='DateTime' not-null='true'>       <column name='CreatedOn' />     </property>     <property name='ModifyDate' column='LastModified' type='DateTime' not-null='true'>       <column name='LastModified' />     </property>     <bag name='Attributes' lazy='true' table='TripItems_TripItemAttributeValues_Link'>       <key column='TripItemId' />       <many-to-many column='TripItemAttributeValueId' class='ETP.Core.Domain.TripItemAttributeValue, ETP.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' />     </bag>   </class> </hibernate-mapping> 

and

<?xml version='1.0' encoding='utf-8'?> <hibernate-mapping xmlns='urn:nhibernate-mapping-2.2' default-lazy='true' assembly='ETP.Core' namespace='ETP.Core.Domain'>   <class name='TripItemAttributeValue' table='TripItemAttributeValues' xmlns='urn:nhibernate-mapping-2.2'>     <id name='Id' column='Id' type='Int32'>       <generator class='identity' />     </id>     <property name='Name' column='Name' length='100' type='String' not-null='true'>       <column name='Name' />     </property>     <bag name='TripItems' lazy='true' table='TripItems_TripItemAttributeValues_Link'>       <key column='TripItemAttributeValueId' />       <many-to-many column='TripItemId' class='ETP.Core.Domain.TripItem2, ETP.Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' />     </bag>   </class> </hibernate-mapping> 

What am I doing wrong here ?

Answer 1

@efdee

I was having the same problem and spent almost two days on this. I had a many to many relationship and the link table wasn’t being updated either. I’m new to NHibernate, just trying to learn it so take everything I say with a grain of salt.

Well it turned out that it’s not Fluent NHibernate, nor the mapping, but me not understanding how NHibernate works with many-to-many. In a many-to-many relationship if collections on both entities aren’t populated, NHibernate doesn’t persist data to the link table.

Let’s say I have this entities in a many-to-many relationship :

 partial class Contact {    public string ContactName {get; set;}    public IList Locations {get; set;}  }  partial class Location {    public string LocationName {get; set;}    public string LocationAddress {get;set;}    public IList Contacts {get;set;} }  

when I add to a location to Contact.Locations, I have to make sure that the contact is also present inside location.Contacts.

so to add a location i have this method inside my Contact class.

 public void AddLocation(Location location)         {             if (!location.Contacts.Contains(this))             {                 location.Contacts.Add(this);             }             Locations.Add(location);         } 

This seems to have solved my problem, but like I said I’m just picking up NHibernate and learning it, may be there’s a better way. If anyone has a better solution, please post.

This is the post that pointed me to check both collections: http://www.coderanch.com/t/217138/Object-Relational-Mapping/link-table-of-ManyToMany-annotation