Question: I have been playing around with google app engine and its datastore recently and created a datamodel and relationships using reference properties. 

However I am unclear about the concept of ancestors wrt the datastore. What is their purpose and why should I use them? How do they relate to reference properties of datastore entities?

Answer: 
Another benefit of entity groups/ancestors is to create islands of strong consistency (as opposed to eventual consistency).

For instance, you could have a project and its tasks. Without ancestors, you could 'close' a task, go back to the tasks list screen for the project, and do a query for open tasks. The task you just closed could still show up because of eventual consistency. The query may have been resolved against a server where the update still hasn't been replicated.

However, with ancestors you get strong consistency. So, instead of a simple foreign key from task to project, you make the the project the ancestor for the project tasks. Now, when querying for tasks, you make it an ancestor query by also providing the project key. The result will be strongly consistent and the task you just closed won't ever be part of the result.
The example usually used is an author and their books. 

Each book is stored in the datastore as a separate entity, with it's name and it's author stored as fields in the model. 

If you want to know all books by a single author, you can run a query such that

```
book.author == desired_author

```

But with ancestors you can also save each book and set a model (a new author model) to be it's parent (it's ancestor). 

So now you can simply say "show me all books that have this author as parent". 

Or rather "show me all children of this ancestor" and all books by that author are returned. 

It might not seem that useful in this example, but if you imagine instead of "author" you have "user" and instead of "book" you have "message board message" and tens of thousands of messages and suddenly it becomes very handy to be able to find all messages by user (e.g. show me my own messages). 

<https://developers.google.com/appengine/docs/python/datastore/queryclass#Query_ancestor>

```
ancestor (ancestor)
Adds an ancestor filter to the query. The query will return only entities with the specified ancestor.

```

There are also benefits in costs for finding each record I expect.