[Bug] Inconsistent model-level and field-level behaviour for sub-objects

[matthetherington]

Describe the bug
With an EditContext model containing one or more sub-objects, eg a Customer which has an Address, the
FieldIdentifier.Model for the changed field will be different to the root object when Address properties have changed. For example, if Address.FirstLine changes, FieldIdentifier.Model will be of type Address, so a registered IValidator<Address> validator will be used to validate at the field-level instead of IValidator<Customer> like at the model-level.

This means an exception is thrown when blurring out of Address fields if an IValidator<Customer> is provided to <FluentValidationValidator> via the Validator property.

If a Validator is not provided, and one is instead retrieved from DI / assembly scan, then any rules affecting the Address that are defined on the IValidator<Customer> validator will not be executed, leading to inconsistent behaviour between model-level and field-level validation.

Also, when validating at the field-level, any configuration provided to the mounted <FluentValidationValidator> component (eg RuleSets) is ignored. The RuleSet will be used for whole-model validation, but ignored for field-level validation, as the configuration is not passed to the validator. At the field-level, rules are executed purely based on the property name, ignoring other configuration.

To Reproduce
I've put together a small Blazor Server app to demonstrate the issues: https://github.com/matthetherington/BlazoredFluentValidationIssueDemo

Expected behavior

• The same model-level validation rules apply to field-level validation, but scoped to the changed property.
• If options are passed to <FluentValidationValidator>, that configuration is used to validate both the whole model and individual fields
• If a Validator is provided to <FluentValidationValidator>, it is possible to validate sub object fields without an exception being thrown
• If a Validator is not provided, the type of EditContext.Model is used to locate a validator instead of FieldIdentifier.Model, so that rules defined on the root-level validator are ran.

In the demo app, I've put together a proposal for how it could behave - pages suffixed with "Fixed" use a modified version of Blazored.FluentValidation with the changes in #205

Screenshots
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Hosting Model (is this issue happening with a certain hosting model?):
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[Albiniivari]

Edit: I tried your PR and it didn't seem to make any difference, so nevermind, the two things are not related.

Hi @matthetherington!
I'm wondering if my issue is related to your bug, if you have the time I'd really appreciate it!

I'm having an issue where I have a model that contains a list of sub-items like such:

class Model {
    List<Item> SelectedItems {get;set;}
     bool IsAccepted {get;set;}
}

class Item{
    string Number {get;set;}
}

I have a case where I have a list of items, and only the selected items should be validated. The selected items have some inputs etc, so they are validated with their own validator. But I have an issue where if I select an item, input bad data so that it gives an error, and then unselect the item. The error doesn't seem to go away. Even when calling editContext.NotifyFieldChange(new FieldIdentifier(model, "SelectedItems")) or editContext.NotifyFieldChange(new FieldIdentifier(item, "Number")).

But if I call editContext.Validate(), it seems to do the trick. But I'd rather just update the one field that needs updating, because the Validate() will show errors for all of the other inputs that the user might've not gotten to yet.

Is this related to your bug ?