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public class ValidationActionFilter : ActionFilterAttribute
{
    // SEE: http://stackoverflow.com/questions/11686690/handle-modelstate-validation-in-asp-net-web-api
    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        var modelState = actionContext.ModelState;

        if (!modelState.IsValid)
        {
            actionContext.Response = actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, modelState);
        }

        //base.OnActionExecuting(actionContext);
    }
}

[HttpPost]
[ValidationActionFilter]
public HttpResponseMessage Approve(SomeModel model)
{
    return this.Request.CreateResponse(HttpStatusCode.OK);
}