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.

There are workarounds to get both the field order and readonly fields right, which comes down to defining MessageAdmin.get_fields() and/or MessageAdmin.get_readonly_fields(), to take full control. However, IMHO this should not be the default.