Adding a custom clean method to your ModelAdmin models

The simplest way is to extend your ModelAdmin model and add a clean() model to it. For example:

from django import forms
from django.db import models

class ModelAdminModel(models.Model):
    def clean(self):
        if self.image.width < 1920 or self.image.height < 1080:
            raise forms.ValidationError("The image must be at least 1920x1080 pixels in size.")

This will run the clean and raise the ValidationError whenever you save the model and the check fails. The error will be displayed at the top of the wagtail admin.

If you want more fine grained-control you can add a custom clean() method to the WagtailAdminPageForm of your model. You can override the form of your ModelAdmin in a similar matter as wagtail Pages.

So, create a custom WagtailAdminPageForm:

from wagtail.admin.forms import WagtailAdminPageForm

class ModelAdminModelForm(WagtailAdminPageForm):
    def clean(self):
        cleaned_data = super().clean()
        image = cleaned_data.get("image")
        if image and image.width < 1920 or image.height < 1080:
            self.add_error("image", "The image must be at least 1920x1080px")

        return cleaned_data

And then set the base_form_class of your model:

from django.db import models

class ModelAdminModel(models.Model):
    base_form_class = ModelAdminModelForm

Using self.add_error will display the error to the particular field that has the error.