Why to always run db:create when restarting containers? - ruby-on-rails

Question

Following https://docs.docker.com/compose/rails/ with a Dockerfile:
FROM ruby:2.3.3
RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs
RUN mkdir /myapp
WORKDIR /myapp
COPY Gemfile /myapp/Gemfile
COPY Gemfile.lock /myapp/Gemfile.lock
RUN bundle install
COPY . /myapp
and a docker-compose.yml:
version: '3'
services:
db:
image: postgres
web:
build: .
command: bundle exec rails s -p 3000 -b '0.0.0.0'
volumes:
- .:/myapp
ports:
- "3000:3000"
depends_on:
- db
it's said:
Stop the application = docker-compose down
Restart the application = docker-compose up AND docker-compose run web rake db:create
Why do we need to create the database every time we restart the containers? Normally we run db:create only once and from then on run db:migrate to run new migrations.

Answer 1

Basically, you have to map your docker local postgresql data files to your own machine.
version: '3'
services:
db:
image: postgres
volumes:
- /var/data/myapp/postgres:/var/lib/postgresql/data:rw
web:
build: .
command: bundle exec rails s -p 3000 -b '0.0.0.0'
volumes:
- .:/myapp
ports:
- "3000:3000"
depends_on:
- db
with this line:
- /var/data/myapp/postgres:/var/lib/postgresql/data:rw
you'll map /var/data/myapp/postgres to your docker /var/lib/postgresql/data and doesn't matter if you down or not your docker-compose, it'll still be available to the next docker-compose up