Django filtering with nested queries

Question

I have a Django model that looks roughly like this

class Equipment(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    name = models.CharField(max_length=100)

class Tag(models.Model):  
    id = models.UUIDField(default=uuid.uuid4, unique=True, primary_key=True)
    tag_type = models.ForeignKey(TagType)
    equipment = models.ForeignKey(Equipment)
    name = models.CharField(max_length=100)

    class Meta:
        db_table = "tag"
        ordering = ["tag_type__name", "name"]
        unique_together = (("tag_type", "equipment"),)

class TagType(models.Model):  
    id = models.PositiveSmallIntegerField(primary_key=True)
    name = models.CharField(max_length=100, unique=True)

    class Meta:
        db_table = "tag_type"
        ordering = ["name"]

TagTypes have names such as "Equipment" and "Load"

"Equipment" tags can be "HVAC" or "Lighting"

"Load" tags can be "Variable" or "Weather"

I want to be able to filter to equipment that have a certain tag_type with a tag name.

If I want equipment that have an HVAC tag with tag_type Equipment the following works: Equipment.objects.filter(tag__tag_type__name="Equipment", tag__name="HVAC")

However, I believe that in this case the tag_type name and the tag aren't necessarily from the same tag. AKA tag__tag_type__name="Equipment" and tag__name="HVAC" might not be referring with the right values.

Since there can be many different types of tags with diverse names, I want to make sure I'm filtering to the right one. Is there a way I can manage this?

Why do you think that the tag_type and the tag_typ_name are not from the same type? — Oded Har-Tal
– Oded Har-Tal, Commented Nov 29, 2016 at 19:01
Well say there is an equipment with two tags associated with it. One tag has tag_type.name=="Equipment" & tag.name=="Fan". The other tag has tag_type.name=="Icon" & tag.name=="HVAC". I believe the filter call I show in my post would have this in the query set when I only want equipment with a tag like tag_type.name=="Equipment" & tag.name=="HVAC". — Eric Blum
– Eric Blum, Commented Nov 29, 2016 at 19:10
So I guess what you'll need is to filter on the Tag table: tags = Tags.objects.filter(type_name = 'Equipement', name = 'HVAC) and take the results [x.equipement for x in tags] — Oded Har-Tal
– Oded Har-Tal, Commented Nov 29, 2016 at 19:13

Eric Blum · Accepted Answer · 2016-12-01 17:54:15Z

1

After some further testing, by chaining tag__tag_type__name="Equipment" and tag__name="HVAC" together ensures that it is indeed looking at the same tag. My worries were unfounded

answered Dec 1, 2016 at 17:54

Eric Blum

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