When doing a simple
gem install pg with the Heroku created Postgress.app, the gem installation fails with the error:
checking for pg_config... no
No pg_config... trying anyway. If building fails, please try again with
The reason is, the pg_config can't be found. What you'll need to do is, install the pg gem with the path that Postgres.app created. The paths can be found on their site: http://postgresapp.com/documentation/
So simply do this:
gem install pg -- --with-pg-config=/Applications/Postgres.app/Contents/MacOS/bin/pg_config
That should be the path where the pg_config is located.