Here’s a snippet from my prototype NHibernateRepository<T> I mentioned the other day. Can you see what I’ve done wrong here?

public abstract class NHibernateRepository<T>
{
    protected ISession unitOfWork;
    protected IEntityValidator<T> validator;

    protected NHibernateRepository(ISession unitOfWork,
        IEntityValidator<T> validator)
    {
        if (unitOfWork == null)
            throw new ArgumentNullException("unitOfWork");
        if (validator == null)
            throw new ArgumentNullException("validator");

        this.validator = validator;
        this.unitOfWork = unitOfWork;
    }

    protected virtual void Validate(T item)
    {
        IEnumerable<RuleViolation> ruleViolations =
            this.validator.GetAllRuleViolations(item);

        if (ruleViolations.Count() == 0)
            return;

        string message = String.Format(
            "The {0} '{1}' could not be persisted because it failed validation.",
            typeof(T).Name, item);

        throw new RuleViolationException(message, ruleViolations);
    }

    public virtual void Save(T item)
    {
        Validate(item);
        using (ITransaction transaction = this.unitOfWork.BeginTransaction())
        {
            this.unitOfWork.SaveOrUpdate(item);
            transaction.Commit();
        }
    }

    // ...
}

See that Validate() method? It’s very clever, automatically checking entities are valid before it commits them to the database. Unfortunately, validating entities is not the responsibility of a repository. This is a big fat violation of the single responsibility principle (SRP), and should be moved to a higher-level domain service instead.

January 21, 2009

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