Feature request: fetch all documents in Reference FIeld list with projection

[halfdanrump]

Hi there!

Using the example class from the docs:

@instance.register
class User(Document):
    email = fields.EmailField(required=True, unique=True)
    birthday = fields.DateTimeField(validate=validate.Range(min=datetime(1900, 1, 1)))
    friends = fields.ListField(fields.ReferenceField("User"))

    class Meta:
        collection = db.user

I'd like to be able to do a query like

friends = User.find_one().friends
friends.fetch_all(projection=['name'])

Does it make sense?

Thank you :)

[halfdanrump]

For the record, I'll be happy to work on this if you think the feature makes sense :)

[lafrech]

IIUC, this is two features.

• Add projection to fetch. Already discussed in https://github.com/Scille/umongo/issues/98. It revealed more complicated than I first thought and I gave it up. I still think it would be a nice feature. For this feature, let's continue discussion there.

• Add fetch_all method to List. Awkward because a ListField can contain any field, not necessarily EmbeddedField.

  With projection in fetch, you could do

  fetched_friends = [f.fetch(projection=['name']) for f in friends]
  

  Not as straightforward but not that bad.

[halfdanrump]

You're right, these are two separate features

With projection in fetch, you could do

fetched_friends = [f.fetch(projection=['name']) for f in friends]

Won't this send n queries to mongo where n is the number of friends?

Leaving aside the projection for now, how about a solution where

User.find_one().friends.fetch_all()

works if and only if the entries in the ListField is a ReferenceField?