Album model -> ModelAdmin error

models.py

from django.db import models
from wagtail.wagtailcore.models import Page, Orderable
from wagtail.wagtailimages.edit_handlers import ImageChooserPanel
from wagtail.wagtailsearch import index
from wagtail.wagtailadmin.edit_handlers import FieldPanel, InlinePanel, MultiFieldPanel
from wagtail.wagtailsnippets.models import register_snippet
from wagtail.wagtailsnippets.edit_handlers import SnippetChooserPanel
from modelcluster.fields import ParentalKey
from modelcluster.models import ClusterableModel
from datetime import datetime


class GenreClassAlbumRelationship(Orderable, models.Model):
    page = ParentalKey(
        'Album', related_name='genre_album_relationship'
    )
    genre = models.ForeignKey(
        'genre.GenreClass',
        related_name="+"
    )
    panels = [
        SnippetChooserPanel('genre')
    ]


class SubGenreClassAlbumRelationship(Orderable, models.Model):
    page = ParentalKey(
        'Album', related_name='subgenre_album_relationship'
    )
    subgenre = models.ForeignKey(
        'genre.SubgenreClass',
        related_name="+"
    )
    panels = [
        # We need this for the inlinepanel on the Feature Content Page to grab hold of
        SnippetChooserPanel('subgenre')
    ]


class AlbumSongs(models.Model):
    album_song = models.CharField("Song name", max_length=255, blank=True)
    album_song_length = models.TimeField("Song length", blank=True)

    panels = [
        FieldPanel('album_song'),
        FieldPanel('album_song_length')
    ]

    class Meta:
        abstract = True


class AlbumSongsRelationship(Orderable, AlbumSongs):
    album_songs = ParentalKey('album', related_name='album_songs_relationship')


class AlbumArtistRelationship(Orderable, models.Model):
    ArtistRelationship = ParentalKey(
        'Album',
        related_name='album_artist_relationship'
    )
    artist_name = models.ForeignKey(
        'artist.Artist',
        #app.class
        related_name="+",
        help_text='The artist(s) who made this album'
    )
    panels = [
        SnippetChooserPanel('artist_name')
    ]


@register_snippet
class Album(ClusterableModel):

    search_fields = Page.search_fields + (
        index.SearchField('artist_name'),
        index.SearchField('biography'),
    )

    album_name = models.CharField("The album's name", blank=True, max_length=254)

    image = models.ForeignKey(
        'wagtailimages.Image',
        null=True,
        blank=True,
        on_delete=models.SET_NULL,
        related_name='+',
        help_text='Album cover image'
    )

    release_date = models.DateField()

    @property
    def album_songs_in_editor(self):
        album_songs_in_editor = [
            n.album_song for n in self.album_songs_relationship.all()
        ]
        return album_songs_in_editor

    @property
    def album_artist(self):
        album_artist = [
            n.artist_name for n in self.album_artist_relationship.all()
        ]
        return album_artist

    @property
    def genre(self):
        genres = [
            n.genre for n in self.genre_album_relationship.all()
        ]
        return genres

    @property
    def subgenres(self):
        subgenres = [
            n.subgenre for n in self.subgenre_album_relationship.all()
        ]
        return subgenres

    panels = [
        FieldPanel('album_name'),
        InlinePanel('album_artist_relationship', label="Arist(s)", panels=None, min_num=1),
        ImageChooserPanel('image'),
        FieldPanel('release_date'),
        MultiFieldPanel(
            [
                InlinePanel('album_songs_relationship', label="Songs for this album", min_num=1),
            ],
            heading="Album songs",
            classname="collapsible"
        ),
        MultiFieldPanel(
            [
                InlinePanel('genre_album_relationship', label="Genre", panels=None, min_num=1, max_num=1),
                InlinePanel('subgenre_album_relationship', label="sub-genres", panels=None, min_num=1),
            ],
            heading="Genres",
            classname="collapsible"
        ),
    ]

    def __str__(self):
        return self.album_name

    def get_context(self, request):
        context = super(Album, self).get_context(request)
        return context

    def artist(obj):
        artist = ','.join([str(i) for i in obj.album_artist])
        return artist

    artist.admin_order_field = 'album_artist_relationship'


    class Meta:
        ordering = ['album_name']
        verbose_name = "Album"
        verbose_name_plural = "Albums"