NHibernate - Map multiple database columns to one object

Question

I have a database (which cannot be changed) with a whole load of values that are stored in two columns:

Currently each column is mapped to one property and my Domain object looks like this:

int? Value1
bool IsValue1RealValue

string Value2
bool IsValue2RealValue

bool? Value3
bool IsValue3RealValue

I want to put those into a ValueType which would hold 2 or 3 values:

-The value (stored in object or in T)
-The boolean 'RealValue'
-And (optionally) the name of the .NET property name

At the moment I have the following NHibernate mapping:

  Component(x => x.Value1, fdf =>
  {
     fdf.Map(f => f.Value).Column("Value1");
     fdf.Map(f => f.IsRealValue).Column("IsValue1RealValue");
  });

Value1 has the following type:

public class FlaggedNullableIntValue
{
    public int? Value;
    public bool IsRealValue;
}

My domain object would then look like:

public class DomainEntity
{
    public virtual FlaggedNullableIntValue Value1;
    public virtual FlaggedStringValue Value2;
    public virtual FlaggedNullableBooleanValue Value3;
}

I wonder if it is a good idea to go further down this road or is there another construct that might better suit my needs?

I don't think I understand. How do you want your model to look like? Where there are those T or objects if the the type of Value is int?? What property name do you want to map? — NOtherDev
– NOtherDev, Commented Sep 26, 2013 at 19:37
I've updated my question. The model would have 1 property for every Value+IsRealValue combination in the database. The value is currently still int, but ideally, the mapped object would be able to handle ints, bools and strings as Type for Value. — Laoujin
– Laoujin, Commented Sep 26, 2013 at 21:13
Are you asking if nHibernate has a feature that converts columns in rows (objects) ? If so, I'm pretty sure it's not possible. You have to map each pair manually. — igelineau
– igelineau, Commented Sep 26, 2013 at 21:18
You can only map one DB column to one property, this is a limitation of NHibernate (if that is a limitation) — Rippo
– Rippo, Commented Sep 27, 2013 at 8:58
@Rippo, actually, using components as @Laoujin demonstrated is one way to map multiple columns to one property. Another way is to define an ICompositeUserType. — Daniel Schilling
– Daniel Schilling, Commented Sep 30, 2013 at 20:37

Daniel Schilling · Accepted Answer · 2013-09-30 20:42:49Z

1

I think you are on exactly the right track. One suggestion - you should be able to make your FlaggedNullable*Value class generic so that you don't have to create lots of different classes.

public class FlaggedNullableValue<T>
    where T : struct
{
    public T? Value;
    public bool IsRealValue;
}

It looks like you're using Fluent NHibernate to specify your mappings. As long as you are consistent in the way you name those column pairs, you should be able to create a convention to ease the burden of mapping all those properties and column names.

answered Sep 30, 2013 at 20:42

Daniel Schilling

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1 Comment

Laoujin Over a year ago

We ended up using the ICompositeUserType (as per your comment). It works pretty well so far.

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