NHibernate: could not initialize a collection: - asp.net-mvc

Question

I am struggling with NHibernate. I have a little ASP MVC website. I managed to setup NHibernate but when I want to retrieve data I get this error:
Additional information: could not initialize a collection: [Lore.Models.DatabaseModels.Statue.Keywords#1]
[SQL: SELECT keywords0_.IdStatueKeyword as IdStatue1_1_, keywords0_.IdKeyword as IdKeyword1_, keyword1_.IdKeyword as IdKeyword4_0_, keyword1_.Name as Name4_0_,
keyword1_.Description as Descript8_4_0_
FROM StatueKeyword keywords0_ left outer join Statue keyword1_ on keywords0_.IdKeyword=keyword1_.IdKeyword
WHERE keywords0_.IdStatueKeyword=?]
Also I am not sure if I implemented the many-to-many relationship right. This is my table structure:
Statue
IdStatue
Name
Keyword
IdKeyword
Name
StatueKeyword
IdStatueKeyword
IdStatue
IdKeyword
Statue Class:
public class Statue
{
public Statue()
{
Keywords = new List<Keyword>();
}
public int IdStatue { get; set; }
public string Name { get; set; }
public IList<Keyword> Keywords { get; set; }
}
Keyword Class
public class Keyword
{
public int IdKeyword { get; set; }
public string Name { get; set; }
}
Statue hbm file:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="Lore.Models.DatabaseModels" assembly="Lore">
<class name="Statue" table="Statue" lazy="false" >
<id name="IdStatue" column="IdStatue">
<generator class="identity" />
</id>
<property name="Name" column="Name" not-null="true" type="System.String" />
<bag name="Keywords" table="StatueKeyword" lazy="false">
<key column="IdStatueKeyword"/>
<many-to-many class="Keyword" column="IdKeyword"/>
</bag>
</class>
</hibernate-mapping>
Keyword hbm file
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="Lore.Models.DatabaseModels" assembly="Lore">
<class name="Keyword" table="Statue" lazy="false" >
<id name="IdKeyword" column="IdKeyword">
<generator class="identity" />
</id>
<property name="Name" column="Name" not-null="true" type="System.String" />
<property name="Description" column="Description" not-null="true" type="System.String" />
</class>
</hibernate-mapping>
I need to make this website for a master thesis so any help is much appreciated!

Answer 1

The issue should/could be in the <key> mapping:
<bag name="Keywords" table="StatueKeyword" lazy="false">
<!-- <key> is representing column where current Statue ID should be searched
while the below one seems to be the ID column of the pairing table
so instead of this
<key column="IdStatueKeyword"/>
use this: -->
<key column="IdStatue"/>
<many-to-many class="Keyword" column="IdKeyword"/>
</bag>
Also check these:
6.3. Collections of Values and Many-To-Many Associations
23.2. Author/Work - documented example with many-to-many
Small cite:
The foreign key from the collection table to the table of the owning class is declared using a <key> element.
Another tip, if you do have an ID column of the pairing table, you should try to use augmented feature:
6.7. Using an <idbag>
Another cite from doc about idbag:
Note that the update performance of an <idbag> is much better than a regular <bag>! NHibernate can locate individual rows efficiently and update or delete them individually, just like a list, map or set.
Finally, I (personally) would descourage you from using many-to-many. My view is, that it is better to avoid it. See: 24. Best Practices (a cite:)
Don't use exotic association mappings.
Good usecases for a real many-to-many associations are rare. Most of the time you need additional information stored in the "link table". In this case, it is much better to use two one-to-many associations to an intermediate link class. In fact, we think that most associations are one-to-many and many-to-one, you should be careful when using any other association style and ask yourself if it is really neccessary.
Maybe later also check these:
Am I doing many to many incorrectly when using fluent nhibernate?
How to create NHibernate HasManyToMany relation
many-to-many with extra columns nhibernate
Nhibernate: How to represent Many-To-Many relationships with One-to-Many relationships?