ManyToManyField.through_fields

Using Django Using the ORM
1

Assuming the following models showing only the relevant porytions

class PlantProfile(Base):
    latin_name = models.CharField(max_length=75, unique=True)
    english_name = models.CharField(max_length=75, blank=True)
    french_name = models.CharField(max_length=75, blank=True)

class ProjectUser(AbstractUser):
    plants = models.ManyToManyField(PlantProfile, through="PlantCollection")

class PlantCollection(models.Model):
    owner = models.ForeignKey(ProjectUser, on_delete=models.CASCADE)
    plants = models.ForeignKey(PlantProfile, on_delete=models.CASCADE)
    details = models.CharField(max_length=125, blank=True)

    class Meta:
        constraints = [
            models.UniqueConstraint(name="unique_plant_owner", fields=["owner", "plants"]),
        ]

How can I query PlantProfile and get the owner (or a boolean) assuming that the current logged in user has the plant in his collection (bool True). I am not sure if I make myself clear.

Output would be 4 fields : latin_name, french_name, english_name, owner

owner would be either true or false. True if plant is part of the collection of the user

2

I’m sorry, I’m not sure I’m understanding what you’re asking for here.

Are you asking for a list of all PlantProfile, with a field indicating whether the current user is related to each instance?

Or are you only looking for a list containing those PlantProfile where the ProjectUser is related to it?

3

I am asking for a list of all PlantProfile that would include the field owner in a column.

4

Screenshot 2024-12-18 at 1.05.09 AM
Screenshot 2024-12-18 at 1.05.09 AM3064Ă—614 110 KB

Maybe the image will help. It shows a list of all the plants (PlantProfile). If a plant is in the PlantCollection of the current user, then in the last column (My Plant) indicate True.

5

You’re still asking for two different things here.

Do you want a column containing the owner(s), or do you want the indicator showing whether the requesting user is related? (These are two different queries.)

6

Sorry Ken, I did not realize it was 2 different queries. I want the indicator showing whether the requesting user is related.

But I would be curious to know how the two queries differ.

7

The best I can do for now is using this strategy within the template, but I am not convinced it’s the most efficient way.

{% for item in object_list  %}
...
 <td>
{% for i in item.plantcollection_set.all %}
{% if i.owner == request.user %}Yes{% endif %}
{% endfor %}
</td>
...
{% endfor %}
8

No, that’s not particularly good - it doesn’t scale well.

The best I’ve been able to come up with so far is this:

plant_profile_query = PlantProfile.objects.annotate(is_owner = 
  Coalesce(
    Subquery(
      PlantCollection.objects.filter(
        owner=request.user, plants=OuterRef('pk')
      ).annotate(owns=Value('Yes')).values('owns')
    ), Value('No')
  )
)

If a PlantCollection exists relating a PlantProfile with a ProjectUser, the filter in the subquery will return the value “Yes”. If the filter fails to find a match, the Coalesce function will return the value “No”.
The is_owner annotated attribute will then have either “Yes” or “No” for each instance of PlantProfile.

1 Like
9

YES!! That’s pretty awesome. It works