Jarvis/jarvis/people/admin.py

from django.contrib import admin
from .models import Gymnast


class GymnastAdmin(admin.ModelAdmin):
    model = Gymnast

    def last_name(self, obj):
        return obj.user.last_name

    def first_name(self, obj):
        return obj.user.first_name

    @admin.display(ordering="user__email", description="Email")
    def email(self, obj):
        if obj.user:
            return obj.user.email

        return None

    def is_active(self, obj):
        return obj.user.is_active

    fields = (
        "last_name",
        "cleaned_last_name",
        "first_name",
        "cleaned_first_name",
        "user",
        "birthdate",
        "gender",
        # "email",
        "is_active",
        "orientation",
        "year_of_practice",
        "email_trainer",
        "informations",
    )

    readonly_fields = ["cleaned_last_name", "cleaned_first_name"]

    list_display = ("last_name", "first_name", "age", "email", "is_active")
    list_filter = ("gender", "user__is_active")
    search_fields = ("last_name", "first_name")


admin.site.register(Gymnast, GymnastAdmin)