Making inlines conditional in the Django admin

As of Django 2.2.2 (current latest version as of this writing), I would use the solution provided earlier by @aggieNick02, which is to override get_inline_instances shown below.

class ThingAdmin(models.ModelAdmin):
    inlines = [MyInline,]

    def get_inline_instances(self, request, obj=None):
        if not obj or obj.date >= today: return []
        return super(ThingAdmin, self).get_inline_instances(request, obj)

I'm posting this new answer because as of April 17th, 2019 in this commit, it looks like the future recommended way to do this would be to instead override the get_inlines method. So in later versions, the solution to this could look like the code below, which allows you to specify different sets of inlines and use them based on a condition.

class ThingAdmin(admin.ModelAdmin):
    model = Thing

    inlines = [inline]
    other_set_of_inlines = [other_inline]

    def get_inlines(self, request, obj):
        if obj.date > datetime.date(2012, 1, 1):
            return self.inlines
        else:
            return self.other_set_of_inlines

Thanks to the comments for a change in 1.4. My implementation here wasn't thread safe either, so it really should have been deleted.

Since get_formsets is passed the object and calls get_inline_instances, we can modify both functions to act on the object.

This should work:

class ThingAdmin(admin.ModelAdmin):
    model = Thing

    inlines = [inline]
    other_set_of_inlines = [other_inline]

    def get_inline_instances(self, request, obj=None):
        #                                    ^^^ this is new
        inline_instances = []

        if obj.date > datetime.date(2012, 1, 1):
            inlines = self.inlines
        else:
            inlines = self.other_set_of_inlines

        for inline_class in inlines:
            inline = inline_class(self.model, self.admin_site)
            if request:
                if not (inline.has_add_permission(request) or
                        inline.has_change_permission(request) or
                        inline.has_delete_permission(request)):
                    continue
                if not inline.has_add_permission(request):
                    inline.max_num = 0
            inline_instances.append(inline)
        return inline_instances

    def get_formsets(self, request, obj=None):
        for inline in self.get_inline_instances(request, obj):
            #                                           ^^^^^ this is new
            yield inline.get_formset(request, obj)