在单个Django查询中执行查找和更新

Question

I have two models: MetaModel and RelatedModel . I want to include the result of a RelatedModel lookup within a MetaModel query, and I'd like to do this within a single DB call.

I've tried to define a 'subquery' QuerySet for use in the main query, but that hasn't worked - it's still making two queries to complete the operation.

Note: I can't use a traditional ForeignKey relationship because the profile_id field is not unique. Uniqueness is a combination of profile_id and channel . This is an aggregation table and profile_id is not guaranteed to be unique across multiple third-party channels.

Any suggestions?

Models:

class Channel(models.Model):
    id = models.AutoField(primary_key=True)
    name = models.CharField(
        max_length=25,
    )

class MetaModel(models.Model):
    profile_id = fields.IntegerField()
    channel = fields.ForeignKey(Channel))
    metadata = fields.TextField()

class RelatedModel(models.Model):
    related_id = fields.IntegerField()
    profile_id = fields.IntegerField()
    channel = fields.ForeignKey(Channel))

Dummy data

channel = Channel("Web site A")
channel.save()

sample_meta = MetaModel(profile_id=1234, channel=channel)
sample_related = RelatedModel(profile_id=1234, related_id=5678, channel=channel)

Query:

# Create a queryset to filter down to the single record we need the `profile_id` for
# I've limited to the only field we need via a `values` operation 
related_qs = RelatedAccount.objects.filter(
    related_id=5678,
    channel=channel
).values_list("profile_id", flat=True)

# I'm doing an update_or_create as there is other data to store, not included for brevity
obj, created = MetaModel.objects.update_or_create(
    profile_id=related_qs.first(),  # <<< This var is the dynamic part of the query
    channel=channel,
    defaults={"metadata": "Metadata is added to a new or existing record."}
)

Answer 1

Regarding your note on uniqueness, you can use unique_together option in Django as described here in the documentation .

class MetaModel(models.Model):
    profile_id = fields.ForeignKey(RelatedModel)
    channel = fields.ForeignKey(Channel)
    metadata = fields.TextField()

    class Meta:
        unique_together = ('profile_id', 'channel')

Then you can change your query accordingly and should solve your problem.