How to query parent entity from child entity in Google App Engine (Python) NDB/Datastore?

google-app-engine google-cloud-datastore python-2.7 app-engine-ndb
fusionstrings picture fusionstrings · Apr 9, 2012 · Viewed 10k times · Source

My question is very fundamental, I want to know straight forward and right way to access attribute values of parent entity from a child in App Engine Python. For example I have following model schema. I am using Python 2.7 and NDB.

class Gallery(ndb.Model):
    category    = ndb.StringProperty()
    title       = ndb.StringProperty()
    subtitle    = ndb.StringProperty()

class Image(ndb.Model):
    blob_key    = ndb.BlobKeyProperty()
    title       = ndb.StringProperty()
    gallery     = ndb.StringProperty()
    is_slider   = ndb.StringProperty()

Here "Gallery" is parent of "Image". They form an entity group Exhibition=>Gallery=>Image. I want to display images from Image model along with gallery detail they belong. I can access child entity from a parent (Image from Gallery) but not vice versa. I don't want to use Image model as StructuredProperty in Gallery model. I am displaying images most of the time from all images based on other flags than gallery (one situation is generating a slideshow from all images if is_slider="yes". so querying from Image directly) but still want to display info of related gallery that's why I want to know how to access parent data.

I feel this is a very generic problem and looking for a simple solution like direct access to parent than going back to top of entity group and query Gallery model with some complex logic. Any help is appreciated.

Answer

mjibson picture mjibson · Apr 9, 2012

Use: image_instance.key.parent().get()

https://developers.google.com/appengine/docs/python/ndb/keyclass#Key_parent

Related questions

Google App Engine: ImportError: No module named appengine.ext

I am trying to write a test for my GAE programme which uses the datastore. Following Google's Documentation, I see that I should be adding the path to my SDK into my PYTHONPATH. I did this using: import sys sys.…

Read More
Google App Engine error: NeedIndexError: no matching index found

I'm having trouble with Google's App engine indexes. When running my app via the GoogleAppEngineLauncher, the app is working fine. When deploying the app, I get the following error: NeedIndexError: no matching index found. The suggested index for this query …

Read More
What is the python for Java's BigDecimal?

In Java, when we program money it is recommended to use the class BigDecimal for money. Is there something similar in python? I would like something object-oriented that can have a currency and an exchange rate, has that been done? …

Read More