What's the difference between a parent and a reference property in Google App Engine?

Question

From what I understand, the parent attribute of a db.Model (typically defined/passed in the constructor call) allows you to define hierarchies in your data models. As a result, this increases the size of the entity group. However, it's not very clear to me why we would want to do that. Is this strictly for ACID compliance? I would like to see scenarios where each is best suited or more appropriate.

Answer 1

There are several differences:

• All entities with the same ancestor are in the same entity group. Transactions can only affect entities inside a single entity group.
• All writes to a single entity group are serialized, so throughput is limited.
• The parent entity is set on creation and is fixed. References can be changed at any time.
• With reference properties, you can only query for direct relationships, but with parent properties you can use the .ancestor() filter to find everything (directly or indirectly) descended from a given ancestor.
• Each entity has only a single parent, but can have multiple reference properties.

Answer 2

The only purpose of entity groups (defined by the parent attribute) is to enable transactions among different entities. If you don't need the transactions, don't use the entity group relationships.

I suggest you re-reading the Keys and Entity Groups section of the docs, it took me quite a few reads to grasp the idea.

Also watch these talks, among other things they discuss transactions and entity groups:

• Building Scalable Web Applications with Google App Engine
• Under the Covers of the Google App Engine Datastore