asp.net mvc database interaction validation

Question

Does anybody have any links or advice on how to hook up validation that requires interacting with the database before updating or adding to the database? Every example I see shows how to validate properties e.g. "Is Required", "Is Email", "Is Numeric", etc, but how do you hook up validation for "Can't order out of stock item"? This xVal blog post touches on it but doesn't provide an example.

I've been following the NerdDinner tutorial which uses a Repository, but this is the bit I don't quite get... Say we had an OrderController with a Create method, and before creating an order we had to first check that the item is in stock. In the NerdDinner style the Controller uses the Repository to talk to the database, so how would our Order object (Model) be able to enforce this validation along with the property validation, as it can't talk to the database?

Thanks for any help

Answer 1

In the NerdDinner tutorial, you can checkout the IsVaild and then the GetRuleViolation methods. Based on your business and database rules, you could use these to check the data you have before you insert it. You could even create an IsValidForInsert Method to check any insert specific rules you need to enforce.

In NerdDinner, the GetRuleViolation allows you to retrieve the violated rules and bubble them up to the interface as you choose.

    public bool IsValid
    {
        get { return (GetRuleViolations().Count() == 0); }
    }

    public IEnumerable<RuleViolation> GetRuleViolations()
    {


        if (CheckDbForViolation)
            yield return new RuleViolation("Database Violation", "SomeField");

        if (String.IsNullOrEmpty(Title))
            yield return new RuleViolation("Title is required", "Title");

        if (String.IsNullOrEmpty(Description))
            yield return new RuleViolation("Description is required", "Description");

        if (String.IsNullOrEmpty(HostedBy))
            yield return new RuleViolation("HostedBy is required", "HostedBy");

 ... etc ...


        yield break;
    }

    public bool CheckDbForViolation()

    {

    /// Do your database work here...

    }

You could take this further and split database code into the repository. The CheckDbForViolation would call the repo for the info and then determine if there was a violation or not. In fact if you are using a repository, I think that would be the preferable way of doing it.