Fluent-nhibernate: discriminate objects to load the hierrachy correctly

Question

Update

Well, as i was told by Stuart Charles and James Gregory, my model was weird enough to begin with. Apparently double-way references between classes while discriminating between the different kinds of relations isn't done, and apparently for a very good reason.

I tore the model down, changed it to a different one using HasManyToMany between Users and Fiches and then i added some coded validations to make sure my Users couldn't be linked to more than one Fiche.

So i guess the answer to this kind of problem is kind of a warning: "Don't do it" ;)

Thank you both for taking time to answer!

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Hello everyone

The path to a correct use of Fluent Nhibernate is a hard one, and since i'm beginning to feel really stumped, i figured it wouldn't hurt to ask for help. Here's the thing.

The culprits

I have the following two classes, Fiche and User. A Fiche must have a User owner , and can have many User followers. On the other hand, a User can optionally be linked back to one and only one Fiche. (The EntityUpdateTrace base class contains a guid id and Create/Modification times, for PreSave/PreUpdate use)

Public Class Fiche
    Inherits EntityUpdateTrace(Of Fiche)

    Private m_Owner As User
    Public Property Owner() As User
        ' snip

    Private m_Followers As IList(Of User)
    Public Property Followers() As IList(Of User)
        ' snip

End Class

Public Class User
    Inherits EntityUpdateTrace(Of User)

    Private m_Fiche As Fiche
    Public Property Fiche() As Fiche
        ' snip

End Class

I then have the following mapping

Public Class FicheMapping
Inherits ClassMap(Of Fiche)
Public Sub New()
    [Not].LazyLoad()
    Id(Function(f As Fiche) f.Id).GeneratedBy.GuidComb()
    References(Of User)(Function(f As Fiche) f.Owner).Not.Nullable()
    HasMany(Of User)(Function(f As Fiche) f.Followers).Inverse()
End Sub
End Class

Public Class UserMapping
    Inherits ClassMap(Of User)
    Public Sub New()
        [Not].LazyLoad()
        Id(Function(a As User) a.Id).GeneratedBy.GuidComb()
        References(Of Fiche)(Function(a As User) a.Fiche).Nullable()
    End Sub
End Class

The question

What the hell am i doing wrong? :) I can't seem to have this model working although it looks deceptively simple.

• Fiche loads all User its related to; sounds logical, and i guess i should discriminate on a specific parameter, but since it would be something entirely database related i don't really like the idea (i'm trying to keep the classes squeaky clean as an exercice in domain hygiene). So if the Fiche veryImportantFiche is created by Alice and followed by Bob, when reloading the followers i find Alice and Bob. Is there a way to discriminate between the two type of User?

• I would like to use a List in the database to keep the followers ordered, but as soon as i change the mapping to

  HasMany(Of User)(Function(f As Fiche) f.Followers).AsList().Inverse()

the following error pops up:

null index column for collection: Followers

The amount of information on this error is not really overwhelming, which leads me to suppose i'm taking the wrong way to this model, which leads me back to the fist question... What the hell am i doing wrong?

Thank you all for reading, and for any insight into this!

Answer 1

The List error happening due to the semantics of lists. It's tricky because bags also get mapped to IList by default, but this is due to the lack of a collection type with "bag" semantics in the .NET framework. In a "bag," conceptually, there is no order because items are just thrown into the bag and removed in any order and could easily be moved around inside the bag. A list, however, is inherently ordered and indexed (think of it as an array with additional semantics, so each element has an unique index value). The error message you are getting indicates that NHibernate wants a column that stores the index value for each element (so it must be unique and, since it's an IList, ant integer value). Since you don't appear to have an index column, you'll have to order them another way, perhaps by adding an ORDER BY clause in your query.

As for the first issue, you might need to rethink how you are working with this. Actually, you might be able to leverage NHibernate filters, but I don't have any experience with them to say how it might work out. Your Followers collection is really all users that are related to the Fiche (all Users that reference Fiche), so it's more like RelatedUsers. Using the Repository Pattern, you could have a method like FindFollowers(Fiche fiche) that would query for all RelatedUsers where the User does not equal the Owner. This pattern might additionally benefit you since you wanted to have the Users be ordered. Something like (C#, Criteria query):

session.CreateCriteria(typeof(Fiche))
    .CreateCriteria("RelatedUsers", JoinType.InnerJoin)
        .Add(Restrictions.Not(Restrictions.Eq("Id", fiche.Owner.Id)))
        .AddOrder(Order.Asc("PropertyNameToOrderBy"))
        .List<User>();

Answer 2

It sounds like you need a many-to-many instead of a one-to-many. That way a single user can be referenced by multiple fiches.

As for the ordering of your collection, AsList won't work unless you have an index column in your table; this is a column that contains a numerical value for each record, with no gaps. If you don't want this behavior, then you'll need to use the order-by attribute.