django - inline - Search for existing record instead of adding a new one

Hi the following code worked for me:

from widgets import ImproveRawIdFieldsFormclass BookInline(admin.TabularInline):    model = Book    raw_id_fields=('shelf',)    extra =1class Shelf(ImproveRawIdFieldsForm):    inlines = [BookInline,]

It creates an admin view where you will se the normal Shelf stuff and the additional inline which is a raw id field and you have the posssibility to add new relations and you can chose from existing objects with the "magnifier" icon, which results in a pop-up of a list of all existing books. Besides chose one Book in the pop-up you can also create new Books there. So from my understanding this solves all your requirements

a better solution for this problem is explained here: one-to-many inline select with django admin

edited for your use-case:

#models.pyclass Book(models.Model):    shelf = models.ForeignKey(Shelf, blank=True, null=True, related_name="in_shelf")#admin.pyclass ShelfForm(forms.ModelForm):    class Meta:        model = Shelf    books = forms.ModelMultipleChoiceField(queryset=Book.objects.all())    def __init__(self, *args, **kwargs):        super(ShelfForm, self).__init__(*args, **kwargs)        if self.instance:            if self.instance.in_shelf:                self.fields['books'].initial = self.instance.in_shelf.all()            else:                self.fields['books'].initial = []    def save(self, *args, **kwargs):            instance = super(ShelfForm, self).save(commit=False)        self.fields['books'].initial.update(shelf=None)        self.cleaned_data['books'].update(shelf=instance)        return instance