Reusing postgresql database from volume in docker-compose - postgresql

Question

When I created volume in Docker using command:
docker volume create pg-data
Then I set up basic postgresql database from postgres image:
docker run --rm -v pg-data:/var/lib/postgresql/data --name pg-docker -e POSTGRES_PASSWORD=docker -p 5433:5432 postgres
Everything worked fine. Database persist and I can even access it directly from the host. I created several roles here like app_user_1.
Now then I wanted to spin up postgresql in container using docker-compose. I shutdown the above postgresql container beforehand.
There I have this settting:
version: '3.7'
services:
db:
image: postgres
volumes:
- pg-data:/var/lib/postgresql/data/
expose:
- 5432
restart: always
environment:
- POSTGRES_PASSWORD=docker
- POSTGRES_USER=postgres
web:
build: .
volumes:
- ./app:/app
ports:
- 8001:8000
environment:
- ENVIRONMENT=dev
- TESTING=0
depends_on:
- db
volumes:
pg-data:
However it seems that even though I mapped the same volume and used same env settings as in docker run command the postgresql instance in container created with docker-compose has no databases and no roles at all.
I get the following error:
psql: error: FATAL: role "postgres" does not exist
or
psql: error: FATAL: role "app_user_1" does not exist
So it seems it behaves as though as it is different instance of postgresql.
When I restarted the first container with docker run everything was there (all the databases and roles).
Any idea why this is happening? How can I reuse the databases from the first container in the docker-compose?

Answer 1

You need to define the volume you wish to use (the one you created manually with docker volume create as external to docker-compose as it was created externally
This is because the volumes created by docker-compose are 'internal' to it, so using ones created by just docker are 'external'. =)
Ref the offical docs at https://docs.docker.com/storage/volumes/#use-a-volume-with-docker-compose
The change to your compose file would be as follows:
...
volumes:
pg-data:
external: true
(Just that last line)
Hope that helps! =)
Additional Note
You can confirm this, by performing a docker volume ls | grep pg-data command which will list all volumes, then only show you the ones referencing 'pg-data'.
On my system where I was testing before I gave my answer, I get the following:
docker volume ls | grep pg-data
local pg-data
local postgresstackoverflow_pg-data
As you can see, the docker volume create one is listed first, as a local volume called 'pg-data', then the docker-compose.yml created one is next prefixed by the naming convention of docker-compose with the directory name that it was in at the time.