Django admin question: correct display of choices field in list_display

In my django admin, I'm trying to configure a custom column display:

My model :

class Batch(models.Model):
    WEIGHT_UNIT = (
        (1, 'G'),
        (1000, 'K'),
        (1000 * 100, 'Q'),
        (1000 * 1000, 'T')
    )

    batch_number = models.CharField(max_length=12, unique=True)
    available_quantity_value = models.DecimalField(max_digits=10, decimal_places=3)
    available_quantity_unit = models.IntegerField(choices=WEIGHT_UNIT)

My model admin :

class BatchAdmin(forms.ModelAdmin):
    list_display = ('batch_number', 'available_quantity_value', 'available_quantity_unit')

If I do this, I have two columns in my admin for the batches list page, and the second columns displays the value taken from the WEIGHT_UNIT constant (K, G, etc.)

Now, I would like to have one single column, displaying something like "10K", "250G",开发者_如何学C etc. So here's my new model admin :

class BatchAdmin(forms.ModelAdmin):
    list_display = ('batch_number', 'get_available_quantity')

and in my model :

def get_available_quantity(self):
    return '%s%s' % (self.available_quantity_value, self.available_quantity_unit)
get_available_quantity.shirt_description = _('Available quantity')

But what I got in my column is something like "101000", "2501", etc. How can I tell django to display the value taken from the choices option?

get_available_quantity_unit_display()