Question

I tried to address a repeated property element using a key, like : ndb.Key(‘Books’, ‘Programming.one’) but this key (the .one part) is not valid.

My example model below is a simplified version of my app’s model. In this example code I have dependencies (using keys) between book chapters and tags :

class Tags(ndb.Model):
    tag = ndb.StringProperty()

example : Tags(id = ‘python’, tag = ‘python’)

class Books(ndb.Model):
    book = ndb.StringProperty)
    chapters = ndb.StringProperty(repeated = True)

example : Books(id = ‘Programming’, book = ‘Programming’, chapters = [‘one’, ‘two’])

class Dependencies(ndb.Model):
    chapter_key = ndb.KeyProperty()
    tag_keys = ndb.KeyProperty(repeated = True)

example :

chapter_key = ndb.Key('Books','Programming.one')
dependency_key = ndb.Key('Dependencies', chapter_key.id())
Dependencies(key = dependency_key, chapter_key = chapter_key, 
                   tag_keys = [ndb.Key('Tags', 'python'), ndb.Key('Tags', 'java')])

Is it possible to address a repeated property using a ndb.Key. In my code example the chapter_key is not valid. Can I use a hook or property subclass to make it work?

To make it work I can combine a valid Book Key with a StringProperty to hold the chapter.

book_key = ndb.Key('Books','Programming')
chapter = 'one'
dependency_key = ndb.Key('Dependencies', book_key.id() + '.' + chapter)
Dependencies(key = dependency_key, book_key = book_key, chapter = chapter, 
                   tag_keys = [ndb.Key('Tags', 'python'), ndb.Key('Tags', 'java')])

But I would like to benifit from a key.

I have the same question for a structured property. For this question the repeated StringProperty chapters is replaced by a repeated StructuredProperty like :

class Chapters(ndb.Model):
    chapter = ndb.StringProperty()
    word_count = ndb.IntegerProperty()

About my example and the use of keys :

I use keys in Dependencies, because keys in dependencies can refer to different kinds. These kinds differ from the Book like kind, because they do not have a repeated property like Book chapters. I use repeated depends_on_keys in my application, instead of chapter_keys.

In the example I also left out the parent keys. The Book like kind can have dependencies, but in my application you cannot find entities, which depend on the Book like kind.

Answer 1

No, you cannot use a key to identify a part of an entity. If you want to reference a part of an entity, you will need to use a key in conjunction with your own scheme for addressing entity parameters.