Thin can't find pg_ext - ruby

Question

I'm trying to run the command
thin -R lat.ru start
in the directory 50web\servers\dev (lat.ru is part of the open source project 50web).
I get the following error:
D:\dev\derek\50web\servers\dev>thin -R lat.ru start
C:/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require': ca
nnot load such file -- 2.1/pg_ext (LoadError)
from C:/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in
`require'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/pg-0.18.4/lib/pg.rb:14:in `rescu
e in <top (required)>'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/pg-0.18.4/lib/pg.rb:3:in `<top (
required)>'
from C:/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:135:in
`require'
from C:/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:135:in
`rescue in require'
from C:/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:144:in
`require'
from C:/Ruby21/lib/ruby/site_ruby/2.1.0/getdb.rb:11:in `<top (required)>
'
from C:/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in
`require'
from C:/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in
`require'
from D:/dev/derek/50web/routes/lat.rb:2:in `<top (required)>'
from C:/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in
`require'
from C:/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in
`require'
from lat.ru:3:in `block in <main>'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/rack-1.6.4/lib/rack/builder.rb:5
5:in `instance_eval'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/rack-1.6.4/lib/rack/builder.rb:5
5:in `initialize'
from lat.ru:1:in `new'
from lat.ru:1:in `<main>'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/thin-1.6.4/lib/rack/adapter/load
er.rb:33:in `eval'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/thin-1.6.4/lib/rack/adapter/load
er.rb:33:in `load'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/thin-1.6.4/lib/thin/controllers/
controller.rb:182:in `load_rackup_config'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/thin-1.6.4/lib/thin/controllers/
controller.rb:72:in `start'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/thin-1.6.4/lib/thin/runner.rb:20
0:in `run_command'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/thin-1.6.4/lib/thin/runner.rb:15
6:in `run!'
from C:/Ruby21/lib/ruby/gems/2.1.0/gems/thin-1.6.4/bin/thin:6:in `<top (
required)>'
from C:/Ruby21/bin/thin:23:in `load'
from C:/Ruby21/bin/thin:23:in `<main>'
I'm using pg 0.18.4, which seems like the most recent version:
D:\dev\derek\50web\servers\dev>gem list pg
*** LOCAL GEMS ***
pg (0.18.4)
D:\dev\derek\50web\servers\dev>gem update pg
Updating installed gems
Nothing to update
Ruby version is 2.1.7p400 (2015-08-18 revision 51632) [i386-mingw32].
How can I fix this error?
Update 1 (02.02.2016 08:08 MSK):
I've to run the same project on a different machine:
C:\usr\dp\dev\derek\50web\servers\dev>ruby lat.ru
C:/Ruby22/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require': cannot load such file -- 2.2/pg_ext (LoadError)
from C:/Ruby22/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
from C:/Ruby22/lib/ruby/gems/2.2.0/gems/pg-0.18.4/lib/pg.rb:14:in `rescue in <top (required)>'
from C:/Ruby22/lib/ruby/gems/2.2.0/gems/pg-0.18.4/lib/pg.rb:3:in `<top (required)>'
from C:/Ruby22/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:128:in `require'
I noticed that Ruby seems to look for the pg_ext file in 2.2/pg_ext directory, whereas in reality it's installed in the C:\Ruby22\lib\ruby\gems\2.2.0\gems\pg-0.18.4\lib directory.

Answer 1

It looks like installation of C-extension part of pg gem is broken.
make sure postgresql development libraries are installed
reinstall pg gem and see if there are no error messages
Reinstallation with -f force flag, or better gem uninstall and gem install anew.

Answer 2

I fixed the error by uninstalling the pg gem and re-installing it using gem install pg --pre command. It's necessary because of a bug, which hasn't been fixed in the release version yet.