ModelAdmin is overruling the fields of ModelForm

[Jurrian Tromp]

It seems that in ModelForm both field_order and Meta.fields ordering is being overruled by the admin change view. When rendering, the form object initially has the correct order, but when admin_form (django.contrib.admin.options:1826) is created, the fieldsets (django.contrib.admin.options:1781) coming from get_fieldsets() overrides the correct order in form.fields.

Something similar in #19514 has been reported before, more or less saying that ModelAdmin order should be leading. The problem with this however is that the reason you have a ModelForm here this way is because you try to do something more specialized. Consider this:

class Message(models.Model):
    some_readonly = models.CharField(...)
    overruled_model_m2m_field = models.ManyToManyField(through=SomeModel, ...)

class MessageForm(forms.ModelForm):
    some_form_field = forms.CharField()
    overruled_model_m2m_field = forms.CharField()

    field_order = ['some_form_field', 'overruled_model_m2m_field']

    class Meta:
        model = Message
        fields = ['some_form_field', 'overruled_model_m2m_field']
        exclude = ['some_readonly']

class MessageAdmin(admin.ModelAdmin):
    readonly_fields = ['some_readonly']
    form = MessageForm

Resulting in the following unexpected behavior:

1. For the fields MessageForm is ignored for the most part, MessageAdmin.get_fields() is de-facto shown.
   1. ModelAdmin readonly_fields are shown, even when they are excluded in the ModelForm.
   2. MessageForm.field_order has no effect when used in an admin view.
2. When a model field has through, it is not shown, as it should be an inline. Although it will be added to the fields again in the form (MessageForm.overruled_model_m2m_field)
   1. This makes it appear "randomly" in the ordering as it is updated to the end of base_fields (django.forms.models:324)

Expected behavior:

1. MessageForm takes precedence over MessageAdmin.
   1. Excluded fields are not shown
   2. ModelForm.Meta.fields and ModelForm.field_order are used in favor of ModelAdmin.get_fields()
2. a. Fields with a through that are also specified in the form itself are ordered correctly according to 1b.