NHibernate joining two table into one entity with a composite key

Question

I have the following data structure :

+---------+
|Resume   |
+---------+
|Id (PK)  |
|IsActive |
|...      |
|..       |
|.        |
+---------+

+--------------------+
|Resume_Translation  |
+--------------------+
|ResumeId (PK, FK)   |
|Language (PK)       |
|Title               |
|Description         |
|...                 |
|..                  |
|.                   |
+--------------------+

So I could have such a data with two joined tables :

+----------------------------------------------------------+
|Id | IsActive | ResumeId | Language | Title | Description |
+----------------------------------------------------------+
|1  | true     | 1        | 'fr'     | 'One' | 'One desc'  |
|1  | true     | 1        | 'pl'     | 'Raz' | 'Raz Opis'  |
|2  | true     | 2        | 'fr'     | 'B'   | 'bla bla'   |
|3  | true     | 3        | 'fr'     | 'C'   | 'C bla bla' |
+----------------------------------------------------------+

From my domain point of view I care only about Resume entity. I don't want to have Resume entity with its collection of Resume_Translations because I would only have one Resume entity with a current translation.

public class Resume
{
    public virtual int Id{ get; protected internal set; }

    public virtual string Language { get; protected internal set; }

    public virtual string Title { get; protected internal set; }

    public virtual string Description { get; protected internal set; }

    public virtual bool IsActive { get; protected internal set; }
}

My current mapping with Fluent NHibernate is as follows :

public class ResumeMap : ClassMap<Resume>
{

    public ResumeMap()
    {
        Table("Resume");
        Id(x => x.Id);
        Map(x => x.IsActive);
        // other properties
        Join("Resume_Translation", m =>
                        {
                            m.Fetch.Join();
                            m.Map(x => x.Language).Length(5);
                            m.Map(x => x.Title).Length(100);
                            m.Map(x => x.Description).Length(200);
                        });
    }
}

I can get what I want from the repository without problem just passing in the WHERE predicate the Id of Resume and the Language I want to.

However I have some problems with Inserting and Updating the values.

My question is: How I would define a mapping that NHibernate Inserts a new record only in Resume_Translation table instead of Updating the record for the current entity ?

So what I want to achieve is if I have in the database the following record :

|2  | true     | 2        | 'fr'     | 'B'   | 'bla bla'   |

Join is good for one to one relationship between tables so if I get this into my entity and I change the language and translation, nhibernate is performing an update and I can understand it. If I try to add a new entity with the same Id by different language and translation, nhibernate yields an error that a key already exists and I understand it also.

So, certainly I'm going down the wrong path, but If some one could point me to the correct solution on how I could achieve a mapping that I want I would greatly appreciate.

Another question, how do you deal with a entities and theirs translations from the business point of view ?

Thanks, in advance for your help.

Thomas

Hi Thomas, I have the same question but I didn't like the solution that much. I am trying to do pretty the same you posted, and in my opinion is is fair more elegant. Any goes with that? — RMalke, Jul 19 '12 at 01:39

score 2 · Answer 1 · answered Jul 12 '11 at 13:25

2

Seems like a one to many relationship to me. I would personally have a collection of ResumeTranslation objects within my Resume object. I would then map this as a standard one to many.

You could then add another property ActiveResumeTranslation to your Resume entity that is representative of your current translation.

answered Jul 12 '11 at 13:25

Cole W

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It's that ResumeTranslation just has no meaning from my domain point of view. That's why the question ;) – Tomasz Jaskuλa Jul 22 '11 at 12:42

score 2 · Answer 2 · answered Jul 12 '11 at 15:18

What about using a dictionary, using the language as a key?

public class ResumeTranslation
{
    public virtual string Title { get; protected internal set; }

    public virtual string Description { get; protected internal set; }
}

public class Resume
{
    public virtual int Id{ get; protected internal set; }

    // language is the key to the translation
    // you may even want to hide the dictionary from the public interface of
    // this class and only provide access to a "current" language.
    public virtual IDictionary<string, ResumeTranslation> Translations { get; private set; }

    public virtual bool IsActive { get; protected internal set; }
}

And map it accordingly as a map with a composite-element (sorry, I'm not using fluent, so don't ask me how it would look like). It would exactly match your database model.

score 2 · Accepted Answer · answered Jul 12 '11 at 15:42

Stefan is on the right track. I've tweaked his suggestion to have a bi-directional association which would make updating a lot easier. One catch with this approach is that you need to manually assign the Resume property of the ResumeTranslation instance when inserting so that NHibernate will properly assign the Resume table key to the ResumeTranslation row. So, given the associations you are mapping, this is how it would look in Fluent NH:

public class ResumeTranslation
{
    public virtual string Title { get; protected internal set; }

    public virtual string Description { get; protected internal set; }

             //Needed for bi-directional association:
    public virtual Resume Resume { get; set; }
}


public class ResumeTranslationMap : ClassMap<ResumeTranslation>
{

    public ResumeTranslationMap()
    {
        Table("ResumeTranslation");
        CompositeId()
            .KeyReference(kp => kp.Resume, "ResumeId")
            .KeyProperty(kp => kp.Language, "Language");

        Map(x => x.Title);
        Map(x => x.Description);
    }
}

public class ResumeMap : ClassMap<Resume>
{

    public ResumeMap()
    {
        Table("Resume");
        Id(x => x.Id);
        Map(x => x.IsActive);
        // other properties

        HasMany(c => c.Translations)
            .Inverse()
            .KeyColumn("id") //May not be required but here for reference
            .Cascade.All();
    }
}

NHibernate joining two table into one entity with a composite key

3 Answers3