FluentNHibernate - Mapping a class to multiple tab

问题:

Sorry for a lengthy question. But it is worth giving all the details so please bear with me through to the end.

I'm working against a legacy database over which I do not have much control. I want to be able to map a class to multiple database tables. Here is how my tables look

Lookup

+--------+--------------+------------+
| Column |   DataType   | Attributes |
+--------+--------------+------------+
| Id     | INT          | PK         |
| Code   | NVARCHAR(50) |            |
+--------+--------------+------------+

Culture

+--------------+--------------+------------+
|    Column    |   DataType   | Attributes |
+--------------+--------------+------------+
| Id           | INT          | PK         |
| Culture_Code | NVARCHAR(10) |            |
+--------------+--------------+------------+

Lookup_t9n

+----------------+---------------+---------------------+
|     Column     |   DataType    |     Attributes      |
+----------------+---------------+---------------------+
| Id             | INT           | PK                  |
| Culture_Id     | INT           | FK to Culture table |
| Localised_Text | NVARCHAR(MAX) |                     |
+----------------+---------------+---------------------+

As you can see, I have a lookup table where all lookups are stored. The display text for a lookup is localized and stored in a separate table. This table has a foreign key to culture table to indicate the culture for which the localized text exists.

My class looks like this

public class Lookup {

    public virtual int Id {get; set;}

    public virtual string Code {get; set;}

    public virtual string DisplayText {get; set;}
}

And my FNH mapping class looks like this

public class LookupMappings : ClassMap<Lookup> {

    public LookupMappings()
    {
        Table("Lookup");
        Id(x => x.Id).Column("Id");
        Map(x => x.Code).Column("Code");

        Join("Lookup_t9n", join => {
            join.Map(x => x.DisplayText).Column("Localised_Text"); //Note this place, my problem is here
        })
    }
}

In the above mapping, in Join part I want to provide some where clause like WHERE Lookup_t9n.Culture_Id = Culture.Culture_Id AND Culture.Culture_Code = System.Threading.Thread.CurrentUICulture.CultureCode.

I know this is not a valid SQL but conveys the intent I hope. Has anyone have any experience of doing such a thing.

I can add a mapping layer where I can have classes that map one-to-one with database tables and then write plain c# to map those classes back to my Lookup class. I have rather done that as an interim solution. I was wondering if I can remove that mapping layer with some smart NH use.

回答1:

I do not have simple answer, like CallThis(). I would like to give you suggestion, based on how we are using similar stuff. The solution is base on the standard mapping, hidding its complexity in C# Entities. It is just a draft of the solution so I'll skip the middle Culture table, and will expect that in Lookup_t9n we do store just a culture name (en, cs...)

Let's have this class

public class Lookup {
    public virtual int Id {get; set;}
    public virtual string Code {get; set;}
                                     // for simplicity skipping null checks    
    public virtual DisplayText { get { return Localizations.First().LocalizedText; } } 
    public virtual IList<Localization> Localizations {get; set;}
}

public class Localization { // mapped to Lookup_t9n
    public virtual string CultureName {get; set;}
    public virtual string LocalizedText {get; set;}
}

Having this, we can map the collection of Localizations as HasMany. It could even be mapped as a component (see example of component mapping)

Now, what we do need is to introduce a filter. Example with Fluent. The essential documentation: 18.1. NHibernate filters.

Simplified mapping

filter:

public class CulturFilter : FilterDefinition
{
  public CulturFilter()
  {
    WithName("CulturFilter")
        .AddParameter("culture",NHibernate.NHibernateUtil.String);
  }

collection:

HasMany(x => x.Localization)
    .KeyColumn("Id")
    ...
    .ApplyFilter<CulturFilter>("CultureName = :culture"))
    .Cascade.AllDeleteOrphan();

Finally, we have to introduce some AOP filter, IInterceptor... which will be triggered each time (needed) and adjust the ISession

session
   .EnableFilter("CulturFilter")
   .SetParameter("culture"
     ,System.Globalization.CultureInfo.CurrentCulture.TwoLetterISOLanguageName);

And now we have Localized string based on current culture, while using standard mapping of localized values as a collection.