I have a website where I allow users to create new Part records. I'm trying to figure out the best way to validate specific fields for uniqueness. I want to make sure that somebody doesn't try to add a Part with PartNumber 1234 if that PartNumber already exists on a different Part.
The Web Application is using Asp.net MVC with fluent nHibernate for mapping my objects to the database. I'm using Castle validation on my view models for things like ValidateNonEmpty, ValidateRange, etc. Should I use the ValidateSelf method to query the repository to see if that part number already exists? Something doesn't feel right about using my Repository on the ViewModel.
Would it be better for me to place that logic on the controller action? That doesn't seem right because I expect my ViewModel to already be Validated at the point (during ModelBind).
Or maybe its none of the above. Thanks for any help on this one.
UPDATE Ok, not sure if this will help, but here is what my Save action looks like for a typical Create Action in my project:
public ActionResult Create(PartViewModel viewModel)
{
//I think I'd like to know if its Valid by this point, not on _repository.Save
if(ModelState.IsValid)
{
try
{
var part = _partCreateViewModelMap.MapToEntity(viewModel);
_repository.Save(part);
return Redirect("~/Part/Details/" + part.Id);
}
catch (Exception e)
{
// skip on down...
}
}
// return view to edit
return View(viewModel);
}
I have been asked this question many times. My friends were worried about whether they can perform data access from the validator code. The answer is simple. If you need to do this, you should do it. Usually we need to do such checks at each level of abstraction. And after all checks you should be ready to catch an exception, caused by unique constraint violation.
If you define a unique constraint within the database, then why not delegate the responsibility to check for whether a unique value already exists to the database? Using NHibernate, you can use the NHibernate.Exceptions.ISQLExceptionConverter
interface to capture and transform known errors relating to constraint violations. You can also use NHibernate.Exceptions.IViolatedConstraintNameExtracter
implementers (see NHibernate.Exceptions.TemplatedViolatedConstraintNameExtracter
) to get at the grubby details of your database exception, and transform it into a user-friendly message, repackage as a validation exception of your chosing and catch it in the relevant controller.
Example of a quick, very specific quick and dirty exception converter from one of my projects:
Imports NHibernate
Imports NHibernate.Exceptions
Imports System.Data.SqlClient
Imports System.Data.Common
Namespace NHibernate
Public Class ConstraintViolationExceptionConverter
Implements ISQLExceptionConverter
Public Function Convert(ByVal adoExceptionContextInfo As Global.NHibernate.Exceptions.AdoExceptionContextInfo) As System.Exception Implements Global.NHibernate.Exceptions.ISQLExceptionConverter.Convert
Dim dbEx As DbException = ADOExceptionHelper.ExtractDbException(adoExceptionContextInfo.SqlException)
If TypeOf dbEx Is SqlException Then
Dim sqlError As SqlException = DirectCast(dbEx, SqlException)
Select Case sqlError.Number
Case 547
Return New ConstraintViolationException(adoExceptionContextInfo.Message, adoExceptionContextInfo.SqlException)
End Select
End If
Return SQLStateConverter.HandledNonSpecificException(adoExceptionContextInfo.SqlException, adoExceptionContextInfo.Message, adoExceptionContextInfo.Sql)
End Function
End Class
End Namespace
Configured through the web.config/nhibernate-configuration/session-factory
property element:
<property name="sql_exception_converter">csl.NHibernate.ConstraintViolationExceptionConverter, csl</property>
Edit: Should probably mention that the converter interface has changed in recent versions of NHibernate, the interface from this example is from NHibernate.dll v2.1.0.4000
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With