Flask-WTF和Flask-SQLAlchemy都是很好用的插件，然而当它们结合到一起后，就不是那么美妙了。

问题的提出

在models.py中定义了一个Article、Category和Tag类：

class Article(db.Model):
    \"\"\"定义文章\"\"\"

    __tablename__ = \'articles\'
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(128), unique=True, index=True)
    # 保存md格式的文本
    content = db.Column(db.Text)
    # 保存html格式的文本
    content_html = db.Column(db.Text)
    # 文章分类
    category_id = db.Column(db.Integer, db.ForeignKey(\'categories.id\'))
    # 文章标签
    tags = db.relationship(
        \'Tag\', secondary=\'article_tag_ref\', backref=\'articles\')
class Category(db.Model):
    \"\"\"文章分类\"\"\"

    __tablename__ = \'categories\'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(128), unique=True)
    articles = db.relationship(\'Article\', backref=\'category\', lazy=\'dynamic\')

class Tag(db.Model):
    \"\"\"文章标签\"\"\"

    __tablename__ = \'tags\'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(128), unique=True)

# 文章和标签的映射表 ，多对多关系
article_tag_ref = db.Table(\'article_tag_ref\',
                           db.Column(\'article_id\', db.Integer,
                                     db.ForeignKey(\'articles.id\')),
                           db.Column(\'tag_id\',  db.Integer,
                                     db.ForeignKey(\'tags.id\'))
                           )

然后在forms.py中定义一个ArticleForm表单

class ArticleForm(Form):

    title = StringField(u\"标题\", validators=[Required()])
    category = QuerySelectField(u\"分类\", query_factory=getUserFactory([\'id\', \'name\']), get_label=\'name\')
    tags = StringField(u\"标签\", validators=[Required()])
    content = PageDownField(u\"正文\", validators=[Required()])
    submit = SubmitField(u\"发布\")

此时在处理表单的时候可以这样：

form = ArticleForm()
if form.validate_on_submit():
    article = Article(title=from.data.title, content=form.data.content,category=form.category.data)
    ...

等等，这样怎么处理form.data.tags？只有像下面这样写了：

\"\"\"
:param tags:
    标签列表，如[u\'测试\',u\'Flask\']
\"\"\"
def str_to_obj(tags):
    r = []
    for tag in tags:
        tag_obj = Tag.query.filter_by(name=tag).first()
        if tag_obj is None:
            tag_obj = Tag(name=tag)
        r.append(tag_obj)
    return r

然后在上面的代码中加入：

 form = ArticleForm()
if form.validate_on_submit():
    article = Article(title=from.data.title, content=form.data.content, category=form.category.data, tags=str_to_obj(form.data.tags))   

这样是不是很难看，像form.data.category就是一个对象，为撒到form.data.tags了就不是了，还要专门写一个函数来坐一个转换？这个时候就有必要扩展WTForms中的表单了。
　

WTForms入门

阅读WTForms文档，关于如何创建一个TagListField，贴一下代码：

class TagListField(Field):
    widget = TextInput()

    def _value(self):
        if self.data:
            return u\', \'.join(self.data)
        else:
            return u\'\'

    def process_formdata(self, valuelist):
        if valuelist:
            self.data = [x.strip() for x in valuelist[0].split(\',\')]
        else:
            self.data = []

简单了看了一下WTForms源码，大致搞清楚了上面代码两个方法的作用：

1. _value The _value method is called by the TextInput widget to provide the value that is displayed in the form. 在初始化表单的时候，就是调用这个方法在表单中渲染数据

2. process_formdata 表单提交时，处理该字段的数据。

编写WTForm扩展

根据上面的代码，将TagListField中的字符串转为models.py中定义的Tag对象即可：

class TagListField(Field):
    widget = TextInput()

    def __init__(self, label=None, validators=None,
                 **kwargs):
        super(TagListField, self).__init__(label, validators, **kwargs)

    def _value(self):
        if self.data:
            r = u\'\'
            for obj in self.data:
                r += self.obj_to_str(obj)
            return u\'\'
        else:
            return u\'\'

    def process_formdata(self, valuelist):
        print \'process_formdata..\'
        print valuelist
        if valuelist:
            tags = self._remove_duplicates([x.strip() for x in valuelist[0].split(\',\')])
            self.data = [self.str_to_obj(tag) for tag in tags]
        else:
            self.data = None

    def pre_validate(self, form):
        pass

    @classmethod
    def _remove_duplicates(cls, seq):
        \"\"\"去重\"\"\"
        d = {}
        for item in seq:
            if item.lower() not in d:
                d[item.lower()] = True
                yield item

    @classmethod
    def str_to_obj(cls, tag):
        \"\"\"将字符串转换位obj对象\"\"\"
        tag_obj = Tag.query.filter_by(name=tag).first()
        if tag_obj is None:
            tag_obj = Tag(name=tag)
        return tag_obj

    @classmethod
    def obj_to_str(cls, obj):
        \"\"\"将对象转换为字符串\"\"\"
        if obj:
            return obj.name
        else:
            return u\'\'

主要就是在process_formdata这一步处理表单的数据，将字符串转换为需要的数据。最终就可以在forms.py中这样定义表单了：

...
class ArticleForm(Form):
    \"\"\"编辑文章表单\"\"\"

    title = StringField(u\'标题\', validators=[Required()])
    category = QuerySelectField(u\'分类\', query_factory=get_category_factory([\'id\', \'name\']), get_label=\'name\')
    tags = TagListField(u\'标签\', validators=[Required()])
    content = PageDownField(u\'正文\', validators=[Required()])
    submit = SubmitField(u\'发布\')
...

在views.py中处理表单就很方便了：

def edit_article():
    \"\"\"编辑文章\"\"\"

    form = ArticleForm()
    if form.validate_on_submit():
        article = Article(title=form.title.data, content=form.content.data)
        article.tags = form.tags.data
        article.category = form.category.data
        try:
            db.session.add(article)
            db.session.commit()
        except:
            db.session.rollback()
    return render_template(\'dashboard/edit.html\', form=form)

代码是不是很简洁了？^_^。。。

当然了写一个完整的WTForms扩展还是很麻烦的。这里只是刚刚入门。可以看官方扩展QuerySelectField的源码。。。

最终效果