When trying to deploy a Ruby on Rails application with capistrano to a new server I was getting an error when bundle tried to install the pg gem. It was failing to build the gem's native extension because it couldn't find the path to pg_config.
The pg error message kindly advised me on how to specify the pg_config path, but I needed bundler to know this and not the gem install command.
With a quick search I came across bundle config. This tool lets you append parameters to the install command of a specific gem. All you need to do is run the following command:
bundle config build.pg --with-pg-config=/usr/local/pgsql/bin/pg_config
Which adds the config information into ~/.bundle/config:
This option could also be passed into the bundle install command, but in the case of the pg gem the path will be the same across applications, so I believe that having it globally in the server is the best option.