How to add a calculated field to a django admin inline - django

Question

I am trying to add a calculated field to a Django Admin Inline. Consider this set of classes:
models.py:
class MyGroup(models.Model):
group_name = models.CharField()
size = models.IntegerField()
class MyUser(models.Model):
user_name = models.CharField()
groups = models.ManyToMany(MyGroup, related_name="users", through=MyMembership)
class MyMembership(models.Model):
group = models.ForeignKey(MyGroup)
user = models.ForeignKey(MyUser)
timestamp = models.DateTimeField(auto_add_now=True)
I want to show the group.size field in the inline. I thought the following code would work (based on this answer):
admin.py:
class MyMembershipInline(admin.TabularInline):
model = MyUser.groups.through
fields = (
"group",
"timestamp",
"size",
)
readonly_fields = (
"timestamp",
"size",
)
def size(self, instance):
return instance.group.size
#admin.register(MyUser):
class MyUserAdmin(admin.ModelAdmin)
fields = ("user_name",)
inlines = (MyMembershipInline,)
But I am getting the following error:
Unknown field(s) (size) specified for MyMembership
Any advice?

Answer 1

While looking for a solution to this problem, I found a solution that worked for me at the following URL:
https://www.boris.co/2013/04/computed-field-on-tabularinline.html
The example given is:
class StatsInline(admin.TabularInline):
model = Stats
fields = ('clicked', 'shown', 'avg')
readonly_fields = ('avg',)
verbose_name = 'Stats'
verbose_name_plural = 'Stats'
can_delete = False
def avg(self, obj):
return float(obj.clicked) / obj.shown if obj.shown else 0
The calculated field needs to be provided in both the fields and readonly_fields attributes.

Answer 2

I wound up having to use a custom ModelForm in the inline. I did it like this:
admin.py:
class MyMembershipInlineForm(forms.ModelForm):
class Meta:
model = MyUser.groups.through
fields = ("group", )
readonly_fields = (
"timestamp",
"size",
)
size = forms.IntegerField(disabled=True)
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
my_membership = self.instance
self.initial = {
"size": my_membership.group.size if my_membership.pk else None
}
class MyMembershipInline(admin.TabularInline):
model = MyUser.groups.through
fields = (
"group",
"timestamp",
"size",
)
form = MyMembershipInlineForm
#admin.register(MyUser):
class MyUserAdmin(admin.ModelAdmin)
fields = ("user_name",)
inlines = (MyMembershipInline,)