Google App Engine: Datastore parent key confusion

Question

I am currently learning more about the Google App Engine datastore, and I have some doubts regarding my understanding of the concept of defining a parent key. Now, here's the code for defining a parent key from the GAE documentation:

def guestbook_key(guestbook_name="default"):
    """Constructs a Datastore key for a Guestbook entity with guestbook_name."""
    return ndb.Key('Guestbook', guestbook_name)

Note: this code is included in the source code of an application which accepts entries from a user and stores it in a datastore and displays them collectively on the homepage.

Now, this is what I understand from this code(please correct me if my understanding of this concept is not what it is supposed to be):

The 'guestbook_key' function defines a parent key, which we have named as 'default', for all the posts that the user submits into the datastore. So basically, all the posts that are submitted by the user are stored in an entity named 'Guestbook', and we define a key for it's parent(which is non-existent) named 'default'.

Please correct me wherever I went wrong with my understanding.

Patrick Costello Patrick Costello · Accepted Answer · 2013-11-05T18:10:14

It really depends on how you use this key. Right now, it is just a name. If you put() it, you are putting a Guestbook type with the name "default".

However, if you are using it as a parent, then you may have code that looks like this:

post = Post(parent=guestbook_key())
post.comment = "This is a new post!"
post.put()

In this case, the new Post object will have the Guestbook object with the name "default" as a parent. Then, you could use an ancestor query to get all Posts for a given guestbook.

The reason you might choose to do this (rather than, for example, have a property on each post with the name of the guestbook) is that it guarantees strongly consistent results. This basically allows all requests to see a consistent view of a guestbook. If you didn't have strongly consistent results, you may see cases where a user writes a post but when they look at the guestbook it doesn't appear.

You are correct that you never actually need to create the Guestbook entity. When you define a parent key, it is actually creating a key where the parent key is a prefix of the child key.

Google App Engine: Datastore parent key confusion

1 Answers