Understanding and Fixing the UNION Query Issue in Django 5.1

The Problem

You're encountering an issue with Django's ORM when trying to use union() with subsequent operations. The error occurs because when you call .values('main') after the union operation, the timestamp column that you're trying to order by is no longer in the result set.

Why This Happens

According to the Django documentation, when working with union(), intersection(), and difference() methods:

1. Only certain operations are allowed on the resulting QuerySet: LIMIT, OFFSET, COUNT(*), ORDER BY, and specifying columns (slicing, count(), exists(), order_by(), and values()/values_list()).

2. When you call .values('main'), you're explicitly selecting only the 'main' field, which removes 'timestamp' from the result set. Then when you try to order by 'timestamp', Django raises an error because that column is no longer available.

Solution

Here are a few approaches to fix your issue:

1. Include the timestamp field in your values() call

PYTHONCopied
def query_with_union(data):
    secondary_qs = SecondaryModel.objects.none()
    for elem in data:
        secondary_qs = secondary_qs.union(
            SecondaryModel.objects.filter(
                value=elem['value'],
                timestamp__gte=elem['timestamp'],
            )
            .order_by('-timestamp')
            .values('main', 'timestamp')[:1]
        )
    
    if len(data) > 1:
        secondary_qs = secondary_qs.order_by('-timestamp')

    # Include timestamp in the values call
    return MainModel.objects.get(pk=secondary_qs.values('main', 'timestamp')[:1].values('main'))

2. Use values_list() with flat=True for the final query

PYTHONCopied
def query_with_union(data):
    secondary_qs = SecondaryModel.objects.none()
    for elem in data:
        secondary_qs = secondary_qs.union(
            SecondaryModel.objects.filter(
                value=elem['value'],
                timestamp__gte=elem['timestamp'],
            )
            .order_by('-timestamp')
            .values('main', 'timestamp')[:1]
        )
    
    if len(data) > 1:
        secondary_qs = secondary_qs.order_by('-timestamp')

    # Use values_list with flat=True to get just the main_id
    main_id = secondary_qs.values_list('main', flat=True)[:1].get()
    return MainModel.objects.get(pk=main_id)

3. Alternative approach using annotations and aggregations

If the union approach continues to be problematic, you might consider a different approach using annotations:

PYTHONCopied
from django.db.models import Max, OuterRef, Subquery

def query_with_annotations(data):
    # Create a subquery to find the latest timestamp for each value
    value_ids = [item['value'] for item in data]
    timestamps = {item['value']: item['timestamp'] for item in data}
    
    # Filter SecondaryModel for the values we're interested in
    filtered_secondary = SecondaryModel.objects.filter(
        value__in=value_ids,
    ).filter(
        # For each value, ensure timestamp is >= the minimum we care about
        timestamp__gte=models.Case(
            *[models.When(value=v, then=models.Value(t)) for v, t in timestamps.items()],
            default=models.Value(min(timestamps.values()))
        )
    )
    
    # For each value, find the latest timestamp
    latest_per_value = filtered_secondary.values('value').annotate(
        latest_timestamp=Max('timestamp')
    )
    
    # Find the records that match these latest timestamps
    latest_records = filtered_secondary.filter(
        models.Q(*[
            models.Q(value=item['value'], timestamp=item['latest_timestamp']) 
            for item in latest_per_value
        ], _connector=models.Q.OR)
    ).order_by('-timestamp')
    
    # Get the main_id from the most recent record
    if latest_records.exists():
        return MainModel.objects.get(pk=latest_records.values_list('main', flat=True).first())
    return None

Why the ORM Complains Only When Calling .values('main')

The error occurs specifically when calling .values('main') because:

1. When you create the union query with .values('main', 'timestamp'), both fields are included in each subquery.
2. After the union, both fields are still available in the result set.
3. When you call .order_by('-timestamp') on the union result, it works because 'timestamp' is still in the result set.
4. But when you subsequently call .values('main'), you're explicitly telling Django to only include the 'main' field in the result set, removing 'timestamp'.
5. The error happens because the ORDER BY clause still references 'timestamp', but that column is no longer in the result set after the .values('main') call.

This behavior is consistent with the documentation which states that you must use the column names from the first QuerySet in methods applied to the resulting QuerySet, and that only certain operations are allowed on the combined QuerySet.

I hope this helps you resolve your issue with the union query!