In Firestore, when you are deleting a doc, you can specify a condition like { exists: true }, in order to avoid deleting the document if it doesn't exist.

In a concurrent scenario, it is possible that we would only like to add a doc to a subcollection if the parent document exists.

For example, imagine the following NoSql structure:

   -comment (doc)
       /likes (subcollection created when the first like is given)
          -like1
          -like2
          ...

Is it possible to do something like

likeRef.create(data, { parent_exists: true });

??

Is the only way to handle this situation with Transactions (reading the parent doc, and throwing an error when .exists() is false)?

I am afraid of this type of situation because if the sub-collection is created at the same time as the container document is deleted, there could be orphan "entities" in the database that may even break the UI if it is not prepared for these extreme cases.