I use Postgres.app to run a local Postgres server while doing development work and it’s great. Postgres.app basically provides a completely contained pg server that’s ready to go out of the box — just launch the app and you’re good to go.

Unfortunately because it’s a self contained app things are installed in non-standard places.

More specifically, while installing the `pg` gem on a system I recently had it fail with this error:

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

 /Users/mario/.rvm/rubies/ruby-2.1.2/bin/ruby extconf.rb
checking for pg_config... no
No pg_config... trying anyway. If building fails, please try again with
 --with-pg-config=/path/to/pg_config
checking for libpq-fe.h... no
Can't find the 'libpq-fe.h header
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers. Check the mkmf.log file for more details. You may
need configuration options.

Provided configuration options:
 --with-opt-dir
 --without-opt-dir
 --with-opt-include
 --without-opt-include=${opt-dir}/include
 --with-opt-lib
 --without-opt-lib=${opt-dir}/lib
 --with-make-prog
 --without-make-prog
 --srcdir=.
 --curdir
 --ruby=/Users/mario/.rvm/rubies/ruby-2.1.2/bin/ruby
 --with-pg
 --without-pg
 --with-pg-config
 --without-pg-config
 --with-pg_config
 --without-pg_config
 --with-pg-dir
 --without-pg-dir
 --with-pg-include
 --without-pg-include=${pg-dir}/include
 --with-pg-lib
 --without-pg-lib=${pg-dir}/lib

extconf failed, exit code 1

First off this error output is awesome. It clearly states that pg_config is missing and the solution is pretty simple — just pass in the path to Postgres.app’s pg_config.

I solved it by reinstalling the gem using:

➜ my_api git:(feature/add-ember) ✗ gem install pg -- --with-pg-config=/Applications/Postgres.app/Contents/Versions/9.4/bin/pg_config
Building native extensions with: '--with-pg-config=/Applications/Postgres.app/Contents/Versions/9.4/bin/pg_config'
This could take a while...
Successfully installed pg-0.17.1
1 gem installed

Bam! As you can see the gem installed successfully!