SQLAlchemy中用==判断None触发参数错误的问题问询

Hey there! Let's work through this query issue you're having. You're trying to pull a list of users linked to specific events, excluding anyone marked with a "dnd" flag (and the flag field can be NULL). The problem is using == None triggers an ArgumentError, and your initial fixes haven't worked—let's sort this out.

Why == None Causes Errors #

Most ORMs (like SQLAlchemy, which your use of is_() suggests you're working with) don't allow direct == None comparisons for database columns. This is because databases use IS NULL instead of = NULL for checking empty values, so the ORM throws an error to prevent invalid SQL generation.

The Correct Way to Filter #

Your goal is to include users who either:

• Have no flag set (flag is NULL), or
• Have a flag that isn't "dnd"

Here's how to write this properly with SQLAlchemy:

from sqlalchemy import or_

# Build your query step by step
user_query = (
    session.query(User)
    .join(Event, User.id == Event.user_id)  # Join your event and user tables
    .filter(Event.id.in_(your_specific_event_ids))  # Filter for your target events
    .filter(or_(User.flag != "dnd", User.flag.is_(None)))  # The key fix!
)

Why Your Previous Attempts Might Have Failed #

• If you only used User.flag.is_(None), that would only include users with no flag set—not users who have a flag that's not "dnd". You need the OR condition to cover both cases.
• Double-check that you're using or_ from SQLAlchemy (not Python's built-in or), since the ORM needs to translate this to valid SQL OR logic.

Alternative for Django ORM (if applicable) #

If you're using Django instead, the syntax is similar but uses Q objects:

from django.db.models import Q

user_query = User.objects.filter(
    event__id__in=your_specific_event_ids,
    Q(flag__isnull=True) | Q(flag != "dnd")
)

That should resolve the ArgumentError and correctly return the users you need!

内容的提问来源于stack exchange，提问作者nektar