How to use custom field for search in django admin

In software, anything is possible... SMH at the accepted answer. You have to override get_search_results.

from django.db.models import Count

class ReportsAdmin(admin.ModelAdmin):
    def investment(self, inst):
        return models.OrderDetail.objects.filter(user=inst.user).distinct().count()

    list_display = ['investment']
    search_fields = ['investment']

    def get_search_results(self, request, queryset, search_term):
        # search_term is what you input in admin site
        search_term_list = search_term.split(' ')  #['apple','bar']

        if not any(search_term_list):
            return queryset, False

        if 'investment' in search_term_list:
           queryset = OrderDetail.objects.annotate(
               user_count=Count('user')
           ).filter(user_count__gte=search_term_list['investment'])

        return queryset, False

Well, this is not allowed:

ModelAdmin.search_fields

Set search_fields to enable a search box on the admin change list page. This should be set to a list of field names that will be searched whenever somebody submits a search query in that text box.

These fields should be some kind of text field, such as CharField or TextField. You can also perform a related lookup on a ForeignKey or ManyToManyField with the lookup API “follow” notation:

You don't have such a field at all (never mind that the field has to be a TextField or CharField). What you actually have is a method in your admin class, which cannot be searched at the database level. Ie what's in the search_fields translates to like '%search_term%' type queries executed at the db.