Why doesn't Docker-compose link the container to the database?

Question

Docker

Good afternoon.
I'm trying to figure out docker-compose.
I did it on one project (rails): one container with an application that I build myself, the second one with a database (postgresql) - everything came together and worked:

web:
  build: .
  volumes:
    - .:/myapp
  ports:
    - "3000:3000"
  links:
    - db
  env_file:
    - '.env.web'
db:
  image: library/postgres:9.4.6
  ports:
    - "5432:5432"
  env_file:
    - '.env.db'

On the second project, I do everything in the same way, I just added one more image with redis:

web:
  build: .
  volumes:
    - .:/voip
  ports:
    - "3000:3000"
  links:
    - redis
    - db
  env_file:
    - '.env.web'
db:
  image: library/postgres:9.4.6
  ports:
    - "5432:5432"
  env_file:
    - '.env.db'
redis:
  image: redis
  ports:
    - "6379:6379"

But when building the container (sudo docker-compose build):

Couldn't create database for {"adapter"=>"postgresql", "encondig"=>"unicode", "pool"=>5, "username"=>"postgres", "database"=>db_production", "host"=>"db", "password"=>"mysecretpassword"}
rake aborted!
PG::ConnectionBad: could not translate host name "db" to address: Name or service not known

What could be the reason?