Fluent NHibernate Mapping a join table without a primary key

Question

I have an existing database that has three tables, two entity tables and a link table. Each of the two entity tables have a primary key, id. One uses an int and one uses a guid. The join table contains two columns, the primary key of each table. So, roughly it looks like this:

Table 1: Question Id - primary key, int

...

Table 2: Asset Id - primary key, guid

...

Table 3: QuestionAsset

Asset - (PK, FK guid)

Question - (PK, FK, int)

I have my classes setup as follows:

public class NQuestion
{
    public virtual int Id { get; set; }
    ...
    public virtual IList<NQuestionAsset> Assets { get; set; }
}

public class NQuestionAsset
{
    public virtual NAsset Asset { get; set; }
    public virtual NQuestion Question { get; set; }
}

public class NAsset
{
    public virtual Guid Id { get; set; }
    ...
}

public class QuestionMap : ClassMap<NQuestion>
{
    public QuestionMap()
    {
        Table("Question");
        LazyLoad();

        Id(q => q.Id).GeneratedBy.Identity().Column("Id");
        ...
        HasManyToMany(x => x.Assets).Table("QuestionAsset")
            .ParentKeyColumns.Add("Asset", "Question")
            .Not.LazyLoad();
     }
 }

 public class QuestionAssetMap : ClassMap<NQuestionAsset>
    {
    public QuestionAssetMap()
    {
        Table("QuestionAsset");
        LazyLoad();

        CompositeId().KeyProperty(a => a.Asset, "Id").KeyProperty(a => a.Question, "QuestionId");

        References(a => a.Asset.Id).Column("Asset");
        References(a => a.Question.QuestionId).Column("Question");
    }
}

public class AssetMap : ClassMap<NAsset>
{
    public AssetMap()
    {
        Table("Asset");
        LazyLoad();

        Id(q => q.Id).GeneratedBy.GuidComb().Column("Id");
        ...
    }
}

I have tried more iterations of relationships than I care to admit to get the link table to populate. How do I define this relationship? I can't change the database.

Right now, with the code above I am getting the following exception:

Could not determine type for: Question.Model.NHibernate.NAsset, Question, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null, for columns: NHibernate.Mapping.Column(Id)

Answer 1

In case,

• the pairing table does not have its own ID column
• has only two columns, foreign keys

We should not introduce the pairing object, and just map both ends directly

public class NQuestion
{
    public virtual int Id { get; set; }
    ...
    //public virtual IList<NQuestionAsset> Assets { get; set; }
    public virtual IList<Asset> Assets { get; set; }

and the HasManyToMany will work

public QuestionMap()
{
    ...
    HasManyToMany(x => x.Assets)
         .Table("QuestionAsset")
         .ParentKeyColumn("Asset")
         .ChildKeyColumn("Question")
         ...;

See more here

• https://github.com/jagregory/fluent-nhibernate/wiki/Fluent-mapping
• http://notherdev.blogspot.cz/2012/01/mapping-by-code-onetomany-and-other.html (second part of the article)