Set IUserType by convention

[EamonHetherton]

I am using a custom type in my code to store small base 10 decimal values without the memory overhead of System.Decimal (4 bytes vs 16 bytes).

public struct SmallDecimal : IEquatable<SmallDecimal>, IEquatable<decimal>, IComparable, IComparable<SmallDecimal>
    {...}

I also have a custom IUserType that maps this type.

I am using ConventionModelMapper to set up a number of conventions including one that inspects the property type and sets the appropriate IUserType

mapper.BeforeMapProperty += (mi, propertyPath, map) =>
{
    var propertyType = propertyPath.LocalMember.GetPropertyOrFieldType();
    if (propertyType.Equals(typeof(SmallDecimal)) || propertyType.Equals(typeof(SmallDecimal?)))
    {
        map.Type(typeof(SmallDecimalUserType), null);
    }
};

This works correctly as long as I explicitly map every property that is SmallDecimal, but what I want is to be able to set up some kind of convention such that any property of type SmallDecimal is automatically mapped as a property with a type SmallDecimalUserType without the explicit mapping. (I have a lot of properties across a lot of classes that are currently automatically mapped as decimal here that I want to change to SmallDecimal)

At the moment, any property I do not explicitly map that has a type of SmallDecimal ends up mapped as a component.

I've never used fluent NHibernate but it appears that I could achieve the above with their UserTypeConvention.

I'd rather stick with the native mapping-by-code Is there a parallel in mapping by code?

[EamonHetherton]

Ok, so as best I can infer what I need to do is to change the behavior of the IsProperty(MemberInfo member) method on the IModelInspector used in ConventionModelMapper.

Two imediate problems here

1. ConventionModelMapper does not contain a constructor that takes an IModelInspector instead it always creates a new SimpleModelInspector
2. SimpleModelInspector implements IModelInspector.IsProperty(MemberInfo member) explicitly so I cannot derive from it to change this behavior, I would need to wrap it in a proxy class.

It doesn't seem controversial to change 1) but i'll continue to investigate how best to proceed to solve 2)

[fredericDelaporte]

Hello,

You have reached the NHibernate bug tracker. Please direct usage questions to the user groups:
http://nhibernate.info/groups.html

From your second comment it seems you also ask for some changes in NHibernate, but that is not clear.

Note that with #702 (NH v5.1) you can register/override default mappings from .Net types to NHibernate types.

Call NHibernate.Type.TypeFactory.RegisterType before building your configuration. It takes an IType, not an IUserType, so wrap it into NHibernate.Type.CustomType.

TypeFactory.RegisterType(
    typeof(SmallDecimal),
    new CustomType(typeof(YourUserType), parameters),
    TypeFactory.EmptyAliases);

(parameters should be null or empty if your type is not also an IParameterizedType.)

If you actually need changes to mapping by code, feel free to submit a PR.

[EamonHetherton]

Hi, thanks for the pointer for registering default type mappings, I've had a look and while it will help somewhat reduce the amount of code I put into the mapper.BeforeMapProperty += (mi, propertyPath, map) => handler, it does not seem to work correctly for the string type if the mapped property has a length; I'm sure I'm probably doing something wrong but can't afford to spend the time to dig a lot deeper when I have a working solution using the mapper.BeforeMapProperty += (mi, propertyPath, map) => handler.

This however does not help with the real issue which is that any non-system type that is not explicitly mapped is automatically mapped as a component when I need it to be mapped as a property with a user type.

public class TestEntity
{
	public int Id { get; set; }
	public SmallDecimal ImplicitMapping { get; set; }
	public SmallDecimal ExplicitMapping { get; set; }
}

public void TestMapping()
{
	TypeFactory.RegisterType(typeof(MappingTest.SmallDecimal), new CustomType(typeof(SmallDecimalUserType), null), TypeFactory.EmptyAliases);
	
	var mapper = new NH.Mapping.ByCode.ConventionModelMapper();
	mapper.Class<MappingTest.TestEntity>(map =>
	{
		map.Id(m => m.Id);
		map.Property(m => m.ExplicitMapping);
	});

	var mapping = mapper.CompileMappingForAllExplicitlyAddedEntities();

	var xml = mapping.AsString();

	Console.WriteLine(xml);
}

<?xml version="1.0" encoding="utf-8"?>
<hibernate-mapping xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" namespace="MappingTest" assembly="MappingTest, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" xmlns="urn:nhibernate-mapping-2.2">
  <class name="TestEntity">
    <id name="Id" type="Int32" />
    <component class="SmallDecimal" name="ImplicitMapping" />
    <property name="ExplicitMapping" />
  </class>
</hibernate-mapping>

Ideally the code above should be able to produce the following mapping:

<?xml version="1.0" encoding="utf-8"?>
<hibernate-mapping xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" namespace="MappingTest" assembly="MappingTest, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" xmlns="urn:nhibernate-mapping-2.2">
  <class name="TestEntity">
    <id name="Id" type="Int32" />
    <property name="ImplicitMapping" />
    <property name="ExplicitMapping" />
  </class>
</hibernate-mapping>

[fredericDelaporte]

it does not seem to work correctly for the string type if the mapped property has a length

Yes, this is meant only for default mapping of a .Net type. As soon as you start specifying any type characteristic, it will not apply.

any non-system type that is not explicitly mapped is automatically mapped as a component

I am not familiar with this part of NHibernate (auto-mapping by code), but in fact this looks to me as quite a sensible default behavior. Maybe there is a way to specify to the auto-mapper how to choose between mapping a class property as a mapping property or as a component, but I do not know. Anyway, this is not the right place to discuss this, please ask on groups instead.