If you are getting a build error from Gem when trying to install the PostgreSQL gem called ‘pg’ and you are using the PostgreSQL App instead of installing PostgreSQL via Brew or some other method, it is because Gem cannot find the PostgreSQL config in the standard locations. To correct this you need to provide Gem with the path to the config file stored within the PostgreSQL App’s internal folder structure.

This can be done as follows:

gem install pg -- --with-pg-config=/Applications/Postgres.app/Contents/Versions/9.4/bin/pg_config

<update: for latest versions of Postgres.app>

gem install pg -- --with-pg-config=/Applications/Postgres.app/Contents/Versions/latest/bin/pg_config

Another way to accomplish this is:

export CONFIGURE_ARGS=”with-pg-include=/Applications/Postgres.app/Contents/Versions/9.4/include/”
gem install pg

Note: the latest version of the PostgreSQL App, at the time of this writing was 9.4, your version may be different. Make sure you change the 9.4 in the config path in the command above to the one you are using or it will not work. You can verify the path by opening Finder, going to the Applications folder, locating Postgres.app, right clicking and selecting ‘show package contents’. This will open up the app’s folder structure in Finder.