Using the reference property has also been called a "virtual join" on this forum. You can actually do the select on the intermediary entity without pulling the referenced entities. This is good if you de- normalize the data that you need for your query out of those referenced entities into your intermediary entity (such as LibraryBook above) to avoid pulling the additional entities in your common read cases. A good discussion of many to many joins in the datastore should mostly discuss how to avoid them and find scalable alternatives. :)