EF, POCO, DbContext and Validating Deletions - asp.net-mvc-3

Question

I'm fairly new to the world of MVC and EF, and I've come quite a ways on my own, but one thing I haven't been able to find online is how people validate for "do not delete" conditions.
I'm using EF4.1 database-first POCO classes generated with the DbContext T4 template. In my partial class files I've already decorated all of my classes with the "IValidatableObject" interface that gets called on changes for my business rules that go beyond the standard MetaData attribute type of validations.
What I need now is a validation that works via the same mechanism (and is, therefore, transparent to the UI and the controller) for checking if deletions are OK. My thought was to create an interface like so:
public interface IDeletionValidation
{
DbEntityValidationResult ValidateDeletion(DbEntityValidationResult validationResults);
}
...and then do this in an override to ValidateEntity in the DbContext...
public partial class MyEntityContext
{
protected override DbEntityValidationResult ValidateEntity(DbEntityEntry entityEntry, IDictionary<object, object> items)
{
DbEntityValidationResult val = base.ValidateEntity(entityEntry, items);
if (entityEntry.State == EntityState.Deleted)
{
IDeletionValidation delValidationEntity = entityEntry.Entity as IDeletionValidation;
if (delValidationEntity != null)
val = delValidationEntity.ValidateDeletion(val);
}
return val;
}
...and then I could implement the IDeletionValidation interface on those classes that need to have a validation done before they can be safely deleted.
An example (not working, see caveat in comments) of the ValidateDeletion code would be...
public partial class SalesOrder : IDeletionValidation, IValidatableObject
{
public DbEntityValidationResult ValidateDeletion(DbEntityValidationResult validations)
{
// A paid SalesOrder cannot be deleted, only voided
// NOTE: this code won't work, it's coming from my head and note from the actual source, I forget
// what class I'd need to add to the DbEntityValidationResult collection for this type of validation!
if (PaidAmount != 0)
validations.Add(new ValidationResult("A paid SalesOrder cannot be deleted, only voided"));
return validations;
}
public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
{
List<ValidationResult> validations = new List<ValidationResult>();
// Verify that the exempt reason is filled in if the sales tax flag is blank
if (!IsTaxable && string.IsNullOrEmpty(TaxExemptReason))
validations.Add(new ValidationResult("The Tax Exempt Reason cannot be blank for non-taxable orders"));
return validations;
}
....
}
Am I on the right track? Is there a better way?
Thanks,
CList
EDIT --- Summary of the one-interface method proposed by Pawel (below)
I think the one-interface way presented below and my way above is a little bit of a chocolate vs. vanilla argument in terms of how you want to do it. Performance should be about the same for large numbers of updates / deletes, and you may want to have your delete validation be a separate interface that doesn't apply to all of your validated classes, but if you want all of your validations in one place here it is...
Mod your DBContext
protected override bool ShouldValidateEntity(DbEntityEntry entityEntry)
{
return entityEntry.Sate == EntityState.Deleted ||
base.ShouldValidateEntity(entityEntry);
}
protected override DbEntityValidationResult ValidateEntity(DbEntityEntry entityEntry, IDictionary<object, object> items)
{
var myItems = new Dictionary<object, object>();
myItems.Add("IsDelete", (entityEntry.State == EntityState.Deleted));
// You could also pass the whole context to the validation routines if you need to, which might be helpful if the
// validations need to do additional lookups on other DbSets
// myItems.Add("Context", this);
return base.ValidateEntity(entityEntry, myItems);
}
Put the deletion-validation in your entity's Validate
public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
{
List<ValidationResult> validations = new List<ValidationResult>();
bool isDelete = validationContext.Items.ContainsKey("IsDelete")
? (bool)validationContext.Items["IsDelete"]
: false;
if (isDelete)
{
if (PaidAmount != 0)
validations.Add(new ValidationResult("You cannot delete a paid Sales Order Line", new string[] { "PaidAmount" }));
return validations;
}
// Update / Add validations!!
// Verify that the exempt reason is filled in if the sales tax flag is blank
if (!IsTaxable && string.IsNullOrEmpty(TaxExemptReason))
validations.Add(new ValidationResult("The Tax Exempt Reason cannot be blank for non-taxable orders"));
return validations;
}
...and in the interest of brevity and only putting all of the check-if-delete code in one place, you could even create an extension method on the ValidationContext class (if you're into that sort of thing) like so...
public static class MyExtensions
{
public static bool IsDelete(this System.ComponentModel.DataAnnotations.ValidationContext validationContext)
{
return validationContext.Items.ContainsKey("IsDelete")
? (bool)validationContext.Items["IsDelete"]
: false;
}
}
...which gives us this for our validation code...
public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
{
List<ValidationResult> validations = new List<ValidationResult>();
if (validationContext.IsDelete())
{
....

Answer 1

I am not really sure why you need a separate interface just for deleted entities. You could pass the entity state (or the EntityEntry object, or the context) to your IValidatableObject.Validate() method by using the items dictionary you pass to the base.ValidateEntity() method. Take a look at the "Custom Validation Sample: Uniqness" section in this blog post http://blogs.msdn.com/b/adonet/archive/2011/05/27/ef-4-1-validation.aspx. This way you could do everything using just one interface - IValidatableObject.
In addition to that - by default EF validates only Added and Modified entities. If you want to validate entities that are in the Deleted state you need to override DbContext.ShouldValidateEntity() method with something like this:
protected override bool ShouldValidateEntity(DbEntityEntry entityEntry)
{
return entityEntry.Sate == EntityState.Deleted ||
base.ShouldValidateEntity(entityEntry);
}