Google NDB: Adding an entity with non existing parent
You can create a key for any entity whether this entity exists or not. This is because a key is simply an encoding of an entity kind and either an id or name (and ancestor keys, if any).
This means that you can store a child entity before a parent entity is saved, as long as you know the parent's id or name. You cannot reassign a child from one parent to another, though.
I am working on a web application based on Google App Engine (Python / Webapp2) and Google NDB Datastore. 
I assumed that if I tried to add a new entity using as parent key the key of a no longer existing entity an exception was thrown. I have instead found the entity is actually created.
Am i doing something wrong? 
I may check before whether the parent still exist through a keys_only query. Does it consume GAE read quotas?