Using `Case().alias` with join

Let’s say I have the following models:

class AttachedFile(Model):
  article = ForeignKeyField(Article)
  file_name = StringField() # this is public
  file_url = StringField() # but this field requires users to be subscribed

class Article(Model):
  # ...
  attached_file = ForeignKeyField(AttachedFile)
  # ...
  
class User(Model):
  # ...

class Subscription(Model):
  # ...
  user = ForeignKeyField(User)
  # ...

So what I’m trying to do is to “hide” the article.attached_file.file_url field if the user doesn’t have a subscription. So:

user_id = 3 # dummy id
sub_cte = (Subscription.select()
  .where(Subscription.user == user_id).cte('sub_cte'))

article_fields = [
  AttachedFile.id,
  AttachedFile.file_name,
  Case(fn.EXISTS(sub_cte.select()), [(False, None)], AttachedFile.file_url).alias("file_url") # <- HERE IS THE PROBLEM
]

target_article = (Article.select(*article_fields)
  .join_from(Article, User)
  .join_from(Article, AttachedFile)
  .with_cte(sub_cte)
  .get_or_none())

if target_article is not None:
  print(target_article.attached_file.file_url)

Now the file_id belongs to target_article not to the target_article.attached_file, and the structure of target_article is not what I want.

I even tried this:

Case(...).alias(AttachedFile.file_url) 
# or 
Case(...).alias('"t1"."file_url"')

But it’s raising an error.

What I CANNOT do:

• not using cte’s
• using subqueries
• manually setting target_article.attached_file.file_url

[coleifer]

So the benefit of using .bind_to() is that you can specify the destination model in a multi-model query regardless of whether the alias corresponds to a field. For example, from the tests:

https://github.com/coleifer/peewee/commit/eeff376be5af089e7d3849284a1c1d71b5df215f#diff-969c0712020e8472d8d78ae32190692cc1e8a3e16dc17f6977c8f87a1328adb7R4896-R4904

In the above, we alias a Case expression to a name that does not correspond to a field, and also specify that it should be applied on the related User instance.

If Alias() is extended to support a field/column as the destination, then we lose this functionality.

I think I’m going to leave it as-is for now, even though it is more verbose in your particular examples.

[coleifer]

Let me think about it for a bit this morning.