Database not persisting in docker - ruby-on-rails

Question

I know I am missing something very basic here. I have see some of the older questions on persisting data using docker, but I think I am following the most recent documentation found here.
I have a rails app that I am trying to run in docker. It runs fine but every time I start it up i get ActiveRecord::NoDatabaseError. After I create the database and migrate it, the app runs fine, until I shut it down and restart it.
here is my docker file:
FROM ruby:2.3.0
RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs
ENV RAILS_ROOT /ourlatitude
RUN mkdir -p $RAILS_ROOT/tmp/pids
WORKDIR $RAILS_ROOT
COPY Gemfile Gemfile
COPY Gemfile.lock Gemfile.lock
RUN gem install bundler
RUN bundle install
COPY . .
and here is my docker-compose.yml file
version: '2'
services:
db:
image: postgres:9.4.5
app:
build: .
environment:
RAILS_ENV: $RAILS_ENV
ports:
- "3000:3000"
command: bundle exec rails s -b 0.0.0.0
volumes:
- .:/ourlatitude/database
depends_on:
- db
the basic flow I am following is this:
export RAILS_ENV=development
docker-compose build
docker-compose up
docker-compose run app rake db:create
docker-compose run app rake db:migrate
at this point the app will be running fine
but then I do this
docker-compose down
docker-compose up
and then I am back to the ActiveRecord::NoDatabaseError
So as I said, I think I am missing something very basic.

Answer 1

It doesn't look like you put your postgres on a volume, you may be missing other persistent data sources in your app container, and it appears you missed some indentation on your app container definition.
version: '2'
services:
db:
image: postgres:9.4.5
volumes:
- postgres-data:/var/lib/postgresql/data
app:
build: .
environment:
RAILS_ENV: $RAILS_ENV
ports:
- "3000:3000"
command: bundle exec rails s -b 0.0.0.0
volumes:
- .:/ourlatitude/database
depends_on:
- db
volumes:
postgres-data:
driver: local
In the example above, the postgres data is stored in a named volume. See the advice on docker hub for more details on persisting data for that application. If you are still losing data, check the output of docker diff $container_id on a container to see what files are changing outside of your volumes that would be lost on a down/up.

Answer 2

I managed to get this to work properly using the following docker-compose.yml file.
version: '2'
volumes:
postgres-data:
services:
db:
image: postgres:9.4.5
volumes:
- postgres-data:/var/lib/postgresql/data
app:
build: .
environment:
RAILS_ENV: $RAILS_ENV
ports:
- "3000:3000"
command: bundle exec rails s -b 0.0.0.0
depends_on:
- db
The key was to add the
volumes:
postgres-data:
which creates the named volume and then the
volumes:
- postgres-data:/var/lib/postgresql/data
under the db section which maps the named volume to the expected location in the container of /var/lib/postgresql/data