Custom method to query external db

In one of my models, 3 values refer to unique ID's in an external db I
want to query for a sum for each ID.
What's the best way to query this external MySQL db?

Example
======
Model:
---------
class Test(models.Model):
name = models.CharField(max_length=10)
id1 = models.PositiveIntegerField(max_length=4)
id2 = models.PositiveIntegerField(max_length=4)
id3 = models.PositiveIntegerField(max_length=4)

DB Query:
---------------
select sum(value) from table_test where id1=%i % (id)

Problem(s):
----------------
How do I implement this the optimal way? I obviously don't want x
model instances to run this query 3 times.
(3 times (for id1, id2, id3) is no problem)

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