Django - 如何编写与此 SQL 查询等效的查询集？ - 使用计数和 FIRST_VALUE 管理重复项

Question

I have Model "A" that both relates to another model and acts as a public face to the actual data (Model "B"), users can modify the contents of A but not of B.

For every B there can be many As, and they have a one to many relation.

When I display this model anytime there's two or more A's related to the BI see "duplicate" records with (almost always) the same data, a bad experience.

I want to return a queryset of A items that relate to the B items, and when there's more than one roll them up to the first entered item.

I also want to count the related model B items and return that count to give me an indication of how much duplication is available.

I wrote the following analogous SQL query which counts the related items and uses first_value to find the first A created partitioned by B.

SELECT *
FROM 
(
  SELECT                         
    COUNT(*) OVER (PARTITION BY b_id) as count_related_items,
    FIRST_VALUE(id) OVER (PARTITION BY b_id order by created_time ASC) as first_filter,                       
    *
  FROM A 
) AS A1 
WHERE 
A1.first_filter = A1.id;

As requested, here's a simplified view of the models:

class CoreData(models.Model):
    title = models.CharField(max_length=500)    

class UserData(models.Model):

    core = models.ForeignKey("CoreData", on_delete=models.CASCADE)
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)

    title = models.CharField(max_length=500)
    

When a user creates data it first checks/creates the CoreData, storing things like the title, and then it creates the UserData, with a reference to the CoreData. When a second user creates a piece of data and it references the same CoreData is when the "duplication" is introduced and why you can roll up the UserData (in SQL) to find the count and the "first" entry in the one to many relation.

Answer 1

Assuming my understanding is correct -

If you are querying from the UserData model the query would look something like this:

Considering CoreData.id = 18

user_data = UserData.objects.filter(core__id=18).
order_by("created_time").annotate(duplicate_count=Count('core__userData', filter(core__id=18))).first()

user_data would be the First object created which is related to the CoreData object. Also,

user_data.duplicate_count will give you the Count of UserData objects that are related to the CoreData object.

Reference Docs on Annotate here

Update:

If you need the list of UserData of specific CoreData you could use

user_data = UserData.objects.filter(core__id=18).
order_by("created_time").annotate(duplicate_count=Count('core__UserData', filter(core__id=18)))