Mounting host volume for mongo data in docker compose container

Question

I'd like to run mongo in a docker container, alongside some others I'm using via docker-compose.

I thought mounting the volume like so would allow for it to persist data outside the container:

db:
  image: mongo
  command: -smallfiles -nojournal
  volumes:
    - ./mongo_data:/data

It seems to create some directories, but there's no files on the host mount. The data files are present on the container.

$ find mongo_data
mongo_data
mongo_data/configdb
mongo_data/db

Answer 1

This seems to work:

db:
  image: mongo:3
#  command: -smallfiles -nojournal
  volumes:
    - ./mongo_data:/data/db

I suspect it's just the change to the volumes value.