How to customize save in django admin inline form?

customize-admin-inline-form

#+TITLE: Customize Save In Django Admin Inline Form


* Background

This is a common case in django ORM.
#+begin_src python
  from django.db import models

  class Author(models.Model):
      name = models.CharField(max_length=255)

  class Book(models.Model):
      name = models.CharField(max_length=255)
      author = models.ForeignKey(Author)
#+end_src

As a CMS, we are also required to provide a feature to create Author and
several Books for him together. So, the =admin= class is here:
#+begin_src python
  from django import forms

  class AuthorAdmin(object):
      form = [AuthorForm,]
      inlines = [BookInline,]
      form_layout = (
          ...
      )

  class BookInline(object):
      model = Book
      form = BookForm
      form_layout = (
          ...
      )

  class AuthorForm(forms.ModelForm):
      model = Author

  class BookForm(forms.ModelForm):
      model = Book
#+end_src

With this =BookInline=, you can get all things done without doubt.
But let't think about a special case:

#+begin_quote
  Assume that there's another Model =Press=, every author belongs to
  a press.

  class Press(models.Model):
      ...

  When creating a author and add/update book to him, you also need
  to create/update the same one for the press, synchronously. May be
  it is a obvious bad design, but just a example.

#+end_quote
So, how to do that?

* HowTo
Straightforward to say, you should customize =FormSet= for =BookInline=
and override these two methods:
- =save_new_objects=
- =save_existing_objects=

So, the =BookInline= will becomes

#+begin_src python
  from django.forms.models import BaseInlineFormSet

  class BookInline(object):
      model = Book
      form = BookForm
      fromset = BookFormSet
      form_layout = (
          ...
      )

  class BookInlineFormSet(BaseInlineFormSet):
      def save_new_objects(self, commit=True):
          saved_instances = super(BookInlineFormSet, self).save_new_objects(commit)
          if commit:
              # create book for press
          return saved_instances

      def save_existing_objects(self, commit=True):
          saved_instances = super(BookInlineFormSet, self).save_existing_objects(commit)
          if commit:
              # update book for press
        return saved_instances
#+end_src

* Why
** Traps
Inertia of thinking, we can override =save= function of =BookForm=
like this:

#+begin_src python
  class BookForm(models.ModelForm):
      ...

      def save(self, commit=True):
          instance = super(BookForm, self).save(commit)
          # do anything you want to sync book for Press
#+end_src

In fact that is a *mistake* I've made before, because =save= function of
=BookForm= instance is called with =commit=False=, it means book
instance is lacking attribute values. So, it can't be copied or do
anything else. I will give some hints to show that how django does it.

#+begin_src python
  # django/forms/models.py

  class BaseInlineFormSet(BaseModelFormSet):
      ...
      def save_new(self, form, commit=True):
          # Use commit=False so we can assign the parent key afterwards, then
          # save the object.
          obj = form.save(commit=False)
          pk_value = getattr(self.instance, self.fk.rel.field_name)
          setattr(obj, self.fk.get_attname(), getattr(pk_value, 'pk', pk_value))
          if commit:
              obj.save()
          # form.save_m2m() can be called via the formset later on if commit=False
          if commit and hasattr(form, 'save_m2m'):
              form.save_m2m()
          return obj
#+end_src

Moreover, it is worth mention that although =obj's= Foreignkey is saved above,
#+begin_src python
  setattr(obj, self.fk.get_attname(), getattr(pk_value, 'pk', pk_value))
#+end_src
you can not access it directly as following at downstream
#+begin_src python
  def save_new_objects(self, commit=True):
      saved_instances = super(BookInlineFormSet, self).save_new_objects(commit)
      if commit:
          book = saved_instances[0]
          author = book.author  # wrong way
      return saved_instances
#+end_src
Because django probably already *cached* a invalid author before.
In order to get a "fresh" =author=, use the primary key value of the
related object as stored in the db field instead.
#+begin_src python
  def save_new_objects(self, commit=True):
      saved_instances = super(BookInlineFormSet, self).save_new_objects(commit)
      if commit:
          book = saved_instances[0]
          author = Author.objects.get(pk=book.author_id)  # correct way
      return saved_instances
#+end_src
About caching ForeignKey in django, see [[https://docs.djangoproject.com/en/dev/topics/db/queries/#one-to-many-relationships][django doc]] for more details.

** Workflow Of Django Admin Inline
#+BEGIN_SRC ditaa :file django-admin-inline-save.png
  +--------------+
  |              |
  | AuthorAdmin  |
  |              |
  +--------------+
  +----------------+ +--------------+
  |                | |              |
  | BookInlinAdmin | |  AuthorForm  |
  |                | |              |
  +----------------+ +--------------+
  +--------------+ +--------------+
  |              | |              |
  |  BookFormSet | |    Author    |
  |              | |              |
  +--------------+ +--------------+
  +--------------+        |
  |              |        |
  |   BookForm   |        |
  |              |        |
  +--------------+        |
  +--------------+        |
  |              |        |
  |     Book     |<-------+
  |              |
  +--------------+
  +----------+
  |          |
  |    DB    |
  |   {s}    |
  +----------+
#+END_SRC