fcm_subscribe.py

import firebase_admin
from firebase_admin import db
from firebase_admin import messaging

# Load Firebase configuration from environment.
firebase_admin.initialize_app()
users_ref = db.reference('users')

# Query for the users whose settings/alerts field is set to `true`.
users = users_ref.order_by_child('settings/alerts').equal_to(True).get()
tokens = [u['token'] for u in users.values() if 'token' in u]

# Split into sublists of length 1000 or less.
max_tokens = 1000
token_sublists = [tokens[i:i+max_tokens] for i in xrange(0, len(tokens), max_tokens)]

for sublist in token_sublists:
    resp = messaging.subscribe_to_topic(sublist, 'releases')
    print('Successfully subscribed {0} tokens'.format(resp.success_count))