Firestore emulator

To work with the Google Firestore emulator you can use the environment variable:

export FIRESTORE_EMULATOR_HOST="localhost:8080"

or specify it as an option using FirestoreDb::with_options().

When FIRESTORE_EMULATOR_HOST is set, the library does not look up the Google credentials and uses a stub token instead, since the emulator does not authenticate the requests. This means you do not need any credentials configured to develop against it. Specifying a token source explicitly, with FirestoreDb::with_options_token_source() for example, still takes precedence.