tapestry5做一个更新页面使用了选用分类的select组件，分类不变的状况下提交报错

我设置页面的之后，如果我换一个分类，提交没问题tapestry5 长度限制，里都升级了，按照以前默认的分类不变提交直接就报错了，里其他字段都升级了tapestry5 长度限制，就是两表关联的字段为空了，不是很理解

页面类

ArticleUpdate.java

package com.bw.pages.article;
import java.util.Date;
import java.util.List;
import org.apache.tapestry5.SelectModel;
import org.apache.tapestry5.ValueEncoder;
import org.apache.tapestry5.annotations.Component;
import org.apache.tapestry5.annotations.PageActivationContext;
import org.apache.tapestry5.annotations.Property;
import org.apache.tapestry5.corelib.components.Errors;
import org.apache.tapestry5.corelib.components.Select;
import org.apache.tapestry5.corelib.components.TextArea;
import org.apache.tapestry5.corelib.components.TextField;
import org.apache.tapestry5.ioc.annotations.Inject;
import com.bw.entities.Article;
import com.bw.entities.Category;
import com.bw.services.dao.jpa.AppJpaDAO;
import com.bw.util.CategoryEncoder;
import com.bw.util.CategorySelectModel;
public class ArticleUpdate {
	@Property
	private Article article;
	
	@Property
	private Category category;
	
	@Property
	private List<Category> categorys;
	@PageActivationContext
	private int id;
	@Inject
	private AppJpaDAO dao;
	@SuppressWarnings("unused")
	@Component
	private Errors errors;
	@SuppressWarnings("unused")
	@Component(parameters = { "value=article.title" })
	private TextField articleTitle;
	@SuppressWarnings("unused")
	@Component(parameters = { "value=article.content" })
	private TextArea articleContent;
	
	@SuppressWarnings("unused")
	@Component(parameters = { "value=category", "encoder=categoryEncoder",
			"model=categoryModel","blankLabel=${article.category.getTitle()}"})
	private Select categorySelect;
	public SelectModel getCategoryModel() {
		return new CategorySelectModel(categorys);
	}
	@SuppressWarnings("rawtypes")
	public ValueEncoder getCategoryEncoder() {
		return new CategoryEncoder(dao);
	}
	void onPrepare(){
		if (article == null) {
			article = dao.findByID(Article.class, id);
		}
		categorys = dao.findWithJpql("select o from _category o order by o.level asc");
	}
	
	Object onSuccess() {
		String s = new java.text.SimpleDateFormat("yyyy-MM-dd hh
					

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