为什么执行form.is_valid()会导致删除不在Django表单中的实例字段? [英] Why executing form.is_valid() cause deleting of the instance fields that aren't in the form in django?

问题描述

假设此视图用于编辑记录:

Suppose this view that is for edit record:

def edit_post(request, slug):
    post = get_object_or_404(Post, slug=slug)
    if request.method == "POST":
        form = AddPostForm(request.POST, request.FILES, instance=post)
        # 1
        if form.is_valid():
            # 2
            new_post = form.save(commit=False)
            new_post.save() 
            return redirect('administrator:view_admin_post')
    ...

现在假设这些:

1. 我具有POST模型中存在的field1.

field1具有默认值，并假定 field1取决于先前.

field1 has default value and suppose that the current value of field1 depends on previous.

在这种情况下，当我在# 1行中写入print(post.field1)时，我具有先前的值，但是当我在其中打印时 行# 2我得到了None！

In this situation when I write print(post.field1) In line # 1 I have previous value but when I print that in line # 2 I got None!

这是怎么回事?我怎么能得到我的post.field1?

What is going on?And how can I have my post.field1?

注意:我知道我可以定义一个中间件变量并将以前的值保存在该变量中.但是有更好的方法吗?

Notice: I know I can define a middleware variable and save the previous value in this variable.But is there a better way to do this?

推荐答案

第一点:.is_valid()不会涉及您的数据库(除非您自定义表单的验证，但是，那么，您将知道为什么使用xD).是Model.save()的调用(直接或通过form.save())覆盖了先前的值.

First point: .is_valid() will NOT touch your database (unless you customize the form's validation, but then, well, you'd know why xD). It's the call to Model.save() (either directly or thru form.save()) that overwrites the previous value.

第二点:如果将模型实例传递给表单(这是您要执行的操作)，则可以通过模板中的{{ form.instance.your_field_name }}访问它(及其字段).

Second point: if you pass a model instance to your form (which is what you do), you can access it (and it's fields) thru {{ form.instance.your_field_name }} in the template.

最后，您可以从ModelForm中完全排除模型字段，也可以将其标记为只读.

And finally, you can either totally exclude a model field from the ModelForm, or mark it as read-only.

所有这些都已被FWIW充分记录，因此，我建议您花一些时间阅读文档，然后再进行离奇而又费解的解决方案"(提示:您绝对不需要中间件来获得原始价值. ，它仍然在您的数据库中-直到您覆盖它为止.

All of this is rather well documented FWIW, so I kindly suggest you take some time reading the doc before going any further with bizarre and convoluted "solutions" (hint: you definitly don't need a middleware to get the original value, it's still in your database - until you overwrite it, that is).

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