delete_collection.rb

require "google/cloud/firestore"

firestore = Google::Cloud::Firestore.new(project_id: project_id)
cities_ref = firestore.col "cities"
query = cities_ref

query.get do |document_snapshot|
  puts "Deleting document #{document_snapshot.document_id}."
  document_ref = document_snapshot.ref
  document_ref.delete
end
puts "Finished deleting all documents from the collection."