NHibernate: nullable one-to-one association..?

Question

This is my first time using NHibernate, and I'm currently in the process off writing mappings and restructuring the objects in my application so they map more cleanly.

I have a situation which looks a bit like this:

class A
{
    // Properties of A
    //..

    // References an instance of B (but can be null)
    public B BInstance { get; set; }
}

class B
{
    // Properties relevant to a particular instance of A
}

Now, this seems like a one-to-one association to me, but I can find no references in the NHibernate documentation as to whether a nullable one-to-one association is actually possible.

Ideally, the "B Table" in my database would have an "A_ID" column. The presence of a row with that A_ID would indicate that the associated instance of A is in a non-null association. If there is no row with that A_ID, it would indicate a null association.

The only way I can think to map this is to map a collection from A (which the application restricts to 1 or 0 elements), but I'm wondering if there's a more obvious solution that I'm missing.

Thanks in advance for your assistance.

To clarify: I'm using vanilla NHibernate, not Fluent.

score 2 · Answer 1 · answered Jan 10 '11 at 03:12

2

You should be able to specify References(x => x.BInstance).Nullable() in your mappings (assuming you are using Fluent nHibernate).

answered Jan 10 '11 at 03:12

Egor Pavlikhin

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Poor assumption :) (This is vanilla NHibernate) – Nick Jan 10 '11 at 03:13
@Nick: you should clarify this info in your question. – VoodooChild Jan 10 '11 at 03:15
2

Additionally, I think this would result in an additional foreign key reference from the A table to the B table (rather than the desired outer-join setup)... – Nick Jan 10 '11 at 03:20
1

@Nick, there is already a similar question then http://stackoverflow.com/questions/244812/nhibernate-one-to-one-mapping-where-second-table-data-can-be-null – Egor Pavlikhin Jan 10 '11 at 03:21
@HeavyWave, I'll have a look. – Nick Jan 10 '11 at 03:28
@HeavyWave - the particulars are a bit different (that question has a bi-directional association). However comments from the accepted answer give some credibility to the idea of using a collection mapping. – Nick Jan 10 '11 at 03:35
Nullable helped me.. my scenario is little different but thanks :) – Shantu Sep 16 '17 at 18:25

score 1 · Accepted Answer · answered Jan 14 '11 at 02:56

1

In the end I changed the model so it more closely mapped to the database schema I was trying to achieve.

I did this by making the one-to-one association bi-directional in the object model. (i.e., The B class also held a reference to its associated A class).

This allowed for the straightforward use of a <one-to-one constrained="true" /> association, which is nullable.

answered Jan 14 '11 at 02:56

Nick

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Are you sure is nullable? Does not seem to work here – user377486 Nov 08 '12 at 13:35

score 0 · Answer 3 · answered Jan 10 '11 at 12:42

0

Nullable one-to-one mapping is possible using a one-to-one element. See http://ayende.com/Blog/archive/2009/04/19/nhibernate-mapping-ltone-to-onegt.aspx for pretty good summary by Ayende.

answered Jan 10 '11 at 12:42

Gerke Geurts

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Felice Pollano · Answer 4 · 2011-01-10T15:10:28.300

0

in the A mapping you should write:

<many-to-one class="B" name="BInstance" column="a_ID" not-null="false"/>

and in the class put the propert BInstance of type "B". not-null="false" is the default value, I wrote just for enforce that B can be null as you requested.

edited Jan 10 '11 at 15:10

answered Jan 10 '11 at 14:33

Felice Pollano

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NHibernate: nullable one-to-one association..?

4 Answers4