My wercker.yml for Rails 4, Ruby 2, Postgresql, Rspec, Heroku deploy and automatic Heroku migrations. Had some issues with it, so hopefully it's helpful to someone.

wercker.yml

box: wercker/ubuntu12.04-ruby2.0.0
services:
    - wercker/postgresql
build:
    steps:
        - bundle-install

        - rails-database-yml:
            service: postgresql

        - script:
            name: echo ruby information
            code: |
                echo "ruby version $(ruby --version) running!"
                echo "from location $(which ruby)"
                echo -p "gem list: $(gem list)"

        - script:
            name: Set up db
            code: bundle exec rake db:schema:load RAILS_ENV=test

        - script:
            name: rspec
            code: bundle exec rspec

deploy:
    steps:
        - heroku-deploy
        - script:
            name: Update database
            code: heroku run rake db:migrate --app $TARGET_NAME

When I cat database from wercker it seems to be correct:

test:
    adapter: postgresql
    encoding: "utf8"
    database: <%= ENV['WERCKER_POSTGRESQL_DATABASE'] %><%= ENV['TEST_ENV_NUMBER'] %>
    username: <%= ENV['WERCKER_POSTGRESQL_USERNAME'] %>
    password: <%= ENV['WERCKER_POSTGRESQL_PASSWORD'] %>
    host: <%= ENV['WERCKER_POSTGRESQL_HOST'] %>
    port: <%= ENV['WERCKER_POSTGRESQL_PORT'] %>

hi @managr actually i have same problem for connecting database. But after some check and test, you just remove service option on rails-database-yml step build.

Hope it helps

My config file is:

ruby: 2.2.3
box: phusion/passenger-ruby22
services:
  - postgres
build:
  steps:
    - bundle-install
    - rails-database-yml
    - script:
        name: echo ruby information
        code: |
            echo "ruby version $(ruby --version) running!"
            echo "from location $(which ruby)"
            echo -p "gem list: $(gem list)"
    - script:
        name: Set up db
        code: bundle exec rake db:schema:load RAILS_ENV=test

    - script:
        name: rspec
        code: bundle exec rspec