Important (save method): i tested this feature on DBFlow 4.0.2, with these configuration i noticed that book_id column is always NULL on Author table, it seems doesn’t work properly, to fix it I have overwritten of save() method of Book entity and set manually the parent entity inside authors (book variable). With this workaround it works.
DBFlow 4 — part 2 : Relations and Queries
Marco Cattaneo

Could you kindly share the workaround, i am having a similar problem in that only the variable with the annotation PrimaryKey is not null, but the remaining variables are null..