Validate title and user name unique combination with xVal

I have a form for create/edit articles. Every article is associated with an user.

After article is publshed the link to each article is composed from user name and article title (and {userName}/{articleTitle} should be unique combination):

/articles/{userName}/{articleTitle}

Article class:

public class Article
{
    public int ArticleId { get; set; }

    [Required(ErrorMessage = "Please enter title")]
    public string Title { get; set; }         

    [Required(ErrorMessage = "Please select a user")]
    public int UserId { get; set; }        
}

View model:

public class ArticleFormViewModel
{
    public Article Article { get; set; }
    public SelectList Users { get; set; }

    public ArticleFormViewModel(Article article, Dictionary<int, string> allUsers)
    {
        Article = article;

        List<SelectListItem> list = new List<SelectListItem>();
        list.Add(new SelectListItem() { Value = "", Text = "Please select a user" });

        foreach (var user in allUsers)
        {
            list.Add(new SelectListItem() { Value = user.Key.ToString(), Text = user.Value });
        }

        Users = new SelectList(list, "Value", "Text", Article.UserId);
    }
}

View:

<div id="validationSummary">
    <%= Html.ValidationSummary("Please correct the errors and try again.") %>
</div>

<% using (Html.BeginForm()) {%>               
    <%= Html.Hidden("ArticleId", Model.Article.ArticleId) %>

    <fieldset>
    <legend>Article</legend>            
    <ul>
        <li>
            <label for="UserId">User: <%= Html.ValidationMessage("UserId", "*")%></label>
            <%= Html.DropDownList("UserId", Model.Users) %>
        </li>
        <li>
            <label for="Title">Title: <%= Html.ValidationMessage("Title", "*") %></label>
            <%= Html.TextBox("Title", Model.Article.Title) %>
        </li>            
    </ul>
    <inp开发者_StackOverflowut type="submit" value="Save" />            
    </fieldset>
<% } %>

<%= Html.ClientSideValidation(typeof(Article))
                .AddRule("Title", new RemoteRule(Url.Action("ValidateTitle")))
                .UseValidationSummary("validationSummary", "Please correct the errors and try again.")%> 

I'm using xVal for validation.

ValidateTitle - is a controller action which validates that {userName}/{articleTitle} is unique. It works using Ajax.

Everything works fine when I'm editing title, but I have problems when I change the user in select list. If title was invalid for user1, and I choose user2, previous error message remains and I can't check that title for user2 is valid.

I can validate user name the same way I do it with title, but there will be cases when 2 errors saying that user name and title combination is invalid will be shown.

Title errors and user errors should be synchronized, but how?

Or maybe there is another way I should work with title and users list?

xVal generates validation rules (for jQuery validation plug-in):

<script type="text/javascript">xVal.AttachValidator(null, 
{"Fields":[{"FieldName":"ArticleId","FieldRules":[{"RuleName":"DataType","RuleParameters":{"Type":"Integer"}}]},
{"FieldName":"Title","FieldRules":[{"RuleName":"Required","RuleParameters":{},"Message":"Please enter title"}, 
{"RuleName":"Remote","RuleParameters":{"url":"/articles/ValidateTitle"}}]},
{"FieldName":"UserId","FieldRules":[{"RuleName":"Required","RuleParameters":{},"Message":"Please select a user"}]}]}, 
{"ValidationSummary":{"ElementID":"validationSummary","HeaderMessage":"Please correct the errors and try again."}})</script>  

Now there is only one condition: a user should be chosen from select list.

I don't know how to hide (reset) title error message when title + user1 is invalid and I chose user2 from list.

And opposite: hide user error message after I chose user and edit title.