Flask 管理 - 使用 on_model_change() 访问旧值

Question

我的目标是当用户更改现有记录的值时执行一些附加操作。

我在文档中找到了 on_model_change() 并编写了以下代码：

def on_model_change(self, form, model, is_created):
    # get old and new value
    old_name = model.name
    new_name = form.name

    if new_name != old_name:
        # if the value is changed perform some other action 
        rename_files(new_name)

我的期望是

model

 参数将代表应用表单中的新值之前的记录。但事实并非如此。相反，我发现 

model

 始终与 

form

 具有相同的值，这意味着 if 语句从未得到满足。 

后来我尝试了这个：

class MyView(ModelView):
    # excluding the name field from the form
    form_excluded_columns = ('name')

    form_extra_fields = {
        # and adding a set_name field instead
        'set_name':StringField('Name')
    }
    ...

def on_model_change(self, form, model, is_created):
    # getting the new value from set_name instead of name 
    new_name = form.set_name
    ...

虽然这解决了我的目标，但也引发了一个问题：

set_name

字段不会预先填充现有名称，即使用户不打算更改名称，也会强制用户键入名称

我还尝试在

db.rollback()

 开始时执行 

on_model_change()

 ，这将撤消 Flask-admin 所做的所有更改，并使 

model

 代表旧数据。这是相当 hacky 的，导致我自己重新实现了很多 Flask 管理代码，这变得很混乱。 

解决这个问题的最佳方法是什么？

我是如何解决的 我使用 on_form_prefill 来预填充新名称字段，而不是 @pjcunningham 的答案。

# fill values from model 
def on_form_prefill(self, form, id):
    # get track model
    track = Tracks.query.filter_by(id=id).first()

    # fill new values
    form.set_name.data = track.name

Answer 1

在您的视图中覆盖方法

update_model。如果您使用 SqlAlchemy 视图，这是默认行为，我添加了一些注释来解释模型的状态。

def update_model(self, form, model):
    """
        Update model from form.
        :param form:
            Form instance
        :param model:
            Model instance
    """
    try:
        # at this point model variable has the unmodified values

        form.populate_obj(model)

        # at this point model variable has the form values

        # your on_model_change is called
        self._on_model_change(form, model, False)

        # model is now being committed
        self.session.commit()
    except Exception as ex:
        if not self.handle_view_exception(ex):
            flash(gettext('Failed to update record. %(error)s', error=str(ex)), 'error')
            log.exception('Failed to update record.')

        self.session.rollback()

        return False
    else:
        # model is now committed to the database
        self.after_model_change(form, model, False)

    return True

您需要类似以下的内容，这取决于您在哪里放置支票，我在模型提交后将其放置：

def update_model(self, form, model):
    """
        Update model from form.
        :param form:
            Form instance
        :param model:
            Model instance
    """
    try:

        old_name = model.name
        new_name = form.name.data

        # continue processing the form

        form.populate_obj(model)
        self._on_model_change(form, model, False)
        self.session.commit()
    except Exception as ex:
        if not self.handle_view_exception(ex):
            flash(gettext('Failed to update record. %(error)s', error=str(ex)), 'error')
            log.exception('Failed to update record.')

        self.session.rollback()

        return False
    else:

        # the model got committed now run our check:
        if new_name != old_name:
            # if the value is changed perform some other action
            rename_files(new_name)

        self.after_model_change(form, model, False)

    return True

您可以为

create_model 和 delete_model 重写类似的方法。

Answer 2

根据 @pjcunningham 的出色推荐，我是这样做的：

class MyModel(ModelView):

    def update_model(self, form, model):
        """Override this method to record something from the model before it's updated"""

        # Make a copy of the old surface location before it's updated, so we can compare it later
        self.old_surface = model.surface

        return super().update_model(form, model)

    def after_model_change(self, form, model, is_created):
        """
        Check to see if the "surface" field has changed, and if so, update the GPS coordinates.
        Do this after the model has changed, so we can start a new database transaction.
        """
        if (
            "surface" in form.data.keys()
            and form.data["surface"] != self.old_model.surface
        ):
            get_or_update_gps(structure_obj=model)