Styjun

In a nutshell: when given the last object for a particular ordering, startAfter() ought to return empty results (I think), but it doesn't. Why not?)

I have a large collection that needs periodic maintenance. I'd like to do it by reading batches, doing batch writes, then carrying on recursively.

I think I've built the correct structure, as follows (this will be cloud code eventually. I'm running it in the browser to avoid the painful build-deploy-test loop using the cloud)

function cleanupFoos(startAfter) 
 let query = db.collection('myCollection').orderBy('createdAt').limit(200)
 if (startAfter) query.startAfter(startAfter)
 return query.get().then(querySnapshot => 
 startAfter = _.last(querySnapshot.docs)
 let batch = db.batch()
 let promises = querySnapshot.docs.map(doc => cleanupFoo(doc, batch))
 return Promise.all(promises).then(() => batch)
 ).then(batch => 
 return batch.commit()
 ).then(() => 
 return startAfter ? cleanupFoos(startAfter) : null
 )


function cleanupFoo(doc, batch) 
 // return a promise


// call it
cleanupFoos()

But this code loops infinitely. Watching in the debugger on a single document collection (myCollection: [ just this one doc ]), I get that single doc on the first invocation. It's also the last doc, so,

startAfter = _.last(querySnapshot.docs) // --> just this one doc 

On the recursive call, I can see that startAfter is set to what I expect: the first (and last) doc. My expectation is that get() should now return an empty querySnapshot. But it returns the same single doc, triggering another round, then another.... That's wrong, right?