Question

What is the “parent key” used in Google App Engine’s Datastore classes and what is it used for?

Answer 1

Reading over this would probably help you.

From the documentation:

To designate an entity’s parent, use the parent argument to the model
class constructor when creating the child entity. The value of this
argument can be the parent entity itself or its key; you can get the
key by calling the parent entity’s key() method. The following example
creates an entity of kind Address and shows two ways of designating an
Employee entity as its parent:

#Create Employee entity
employee = Employee()
employee.put()

#Set Employee as Address entity 's parent directly...
address = Address(parent=employee)

# ...or using its key
e_key = employee.key()
address = Address(parent=e_key)

# Save Address entity to datastore
address.put()​