How to install PostgreSQL's pg gem on Ubuntu?

I'm trying to install PostgreSQL's pg gem for Ruby.

I issued the following command:

gem install pg

I installed Ruby 1.9.2 using RVM.

The above command shows me the following error.

The error is :

Building native extensions.  This could take a while...

ERROR:  Error installing pg:

ERROR: Failed to build gem native extension.

/home/User/.rvm/rubies/ruby-1.9.2-preview3/bin/ruby extconf.rb

checking for pg_config... yes
checking for libpq-fe.h... yes
checking for libpq/libpq-fs.h... yes
checking for PQconnectdb() in -lpq... no
checking for PQconnectdb() in -llibpq... no
checking for PQconnectdb() in -lms/libpq... no
Can't find the PostgreSQL client library (libpq)

*** 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=/home/User/.rvm/rubies/ruby-1.9.2-preview3/bin/ruby
 --with-pg
 --without-pg
 --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
 --enable-static-build
 --disable-static-build
 --with-pqlib
 --without-pqlib
 --with-libpqlib
 --without-libpqlib
 --with-ms/libpqlib
 --without-ms/libpqlib

Gem files will remain installed in /home/user/.rvm/gems/ruby-1.9.2-preview3/gems/pg-0.9.0 for inspection.

Results logged to /home/user/.rvm/gems/ruby-1.9.2-preview3/gems/pg-0.9.0/ext/gem_make.out

I don't know what is the error...

You need install the postgreSQL dev package with header of PostgreSQL

sudo apt-get install libpq-dev

After reading and thrashing around for 2 days, and trying many things found in other notes the following single line was the cure for me on Ubuntu Lucid 10.04 mixed with some Maverick packages and RVM (ruby 1.9.2-p290, rvm 1.10.2 rubygems 1.8.15, rails 3.0.1, postgres 8.4.10) :

gem install pg  --   --with-pg-lib=/usr/lib   

the result:

Building native extensions.  This could take a while...  
Successfully installed pg-0.13.1  
1 gem installed  
Installing ri documentation for pg-0.13.1...  
Installing RDoc documentation for pg-0.13.1...  

{yea - finally success} !! !note that the output from running pg_config lacks the item -lpq in the LIBS variable on my Ubuntu / Postresql install!!

and why the switch from pq to pg in certain places -- confusing to newbie ??

the thing I still do not understand is the double set of -- and --with(option but I'm way beyond my depth anyway

In Ubuntu this works for me, I hope help you:

sudo apt-get install libpq-dev

and

gem install pg  --   --with-pg-lib=/usr/lib 

I'm on Ubuntu 12.10 and running this command:

apt-get install libpq-dev

helped me - after that ran gem install pg -v "0.14.1", and all good now

Need to add package

sudo apt-get install libpq-dev

to install pg gem in RoR