I like the presentation of ideas. Can you expand on use cases of Firebase’s nested document model? Seems powerful as a file system but not sure how that will play with the complexities of distributed applications.
As these things tend to go, when there is a place to store arbitrary data, all kinds of things get shoved into it, so the mixed model in Firestore is a compromise between the original tree-of-nodes data model and a more conventional document data model.
My assumption is the Firestore-to-Spanner mapping creates subcollections as shared tables with foreign keys to the parent documents, but I don't actually know. However, that would match the mandatory 1-to-many-to-1-to-many data layout, and makes more sense than shoving all the dependent data into the document itself or creating multiple millions of SQL tables for millions of documents.