Can't find the 'libpq-fe.h header when trying to install pg gem

Question

I am using the Ruby on Rails 3.1 pre version. I like to use PostgreSQL, but the problem is installing the pg gem. It gives me the following error:

Answer 1

I am running Postgres.app on a Mac and I had to

export PATH=$PATH:/Applications/Postgres.app/Contents/Versions/9.4/bin

first. Then

bundle install

worked for me.