Adapters not working with datamapper

Question

I have the following code:

require 'sinatra'
require 'datamapper'

DataMapper.setup :default, "postgres://localhost/mydb"

However, when I try and run it, I get:

LoadError: no such file to load -- dm-postgres-adapter

/Library/Ruby/Site/1.8/rubygems/custom_require.rb:53:in `gem_original_require'

/Library/Ruby/Site/1.8/rubygems/custom_require.rb:53:in `require'

/Library/Ruby/Gems/1.8/gems/dm-core-1.1.0/lib/dm-core/adapters.rb:163:in `load_adapter'

/Library/Ruby/Gems/1.8/gems/dm-core-1.1.0/lib/dm-core/adapters.rb:133:in `adapter_class'

/Library/Ruby/Gems/1.8/gems/dm-core-1.1.0/lib/dm-core/adapters.rb:13:in `new'

/Library/Ruby/Gems/1.8/gems/dm-core-1.1.0/lib/dm-core.rb:219:in `setup'

These are the gems I have installed:

* LOCAL GEMS *

abstract (1.0.0)

actionmailer (3.0.9, 3.0.5, 2.3.5, 1.3.6)

actionpack (3.0.9, 3.0.5, 2.3.5, 1.13.6)

actionwebservice (1.2.6)

activemodel (3.0.9, 3.0.5)

activerecord (3.0.9, 3.0.5, 2.3.5, 1.15.6)

activeresource (3.0.9, 3.0.5, 2.3.5)

activesupport (3.0.9, 3.0.5, 2.3.5, 1.4.4)

acts_as_ferret (0.4.3)

addressable (2.2.6, 2.2.5)

arel (2.0.10, 2.0.9)

autotest (4.4.6)

autotest-fsevent (0.2.4)

autotest-growl (0.2.9)

autotest-rails-pure (4.1.2)

bcrypt-ruby (2.1.4)

builder (2.1.2)

bundler (1.0.10)

capistrano (2.5.2)

cgi_multipart_eof_fix (2.5.0)

configuration (1.2.0)

daemons (1.0.10)

data_objects (0.10.6)

datamapper (1.1.0)

diff-lcs (1.1.2)

dm-aggregates (1.1.0)

dm-constraints (1.1.0)

dm-core (1.2.0.rc2, 1.1.0)

dm-do-adapter (1.2.0.rc2, 1.1.0)

dm-migrations (1.1.0)

dm-postgres-adapter (1.2.0.rc2, 1.1.0)

dm-serializer (1.1.0)

dm-sqlite-adapter (1.2.0.rc2, 1.1.0)

dm-timestamps (1.1.0)

dm-transactions (1.1.0)

dm-types (1.1.0)

dm-validations (1.1.0)

dnssd (0.6.0)

do_postgres (0.10.6)

do_sqlite3 (0.10.6)

erubis (2.6.6)

eventmachine (0.12.10)

ezcrypto (0.7.2)

faraday (0.6.1)

faraday_middleware (0.6.3)

fastercsv (1.5.4)

fastthread (1.0.1)

fcgi (0.8.7)

ferret (0.11.6)

gem_plugin (0.2.3)

hashie (1.0.0)

heroku (1.18.1)

highline (1.5.0)

hpricot (0.8.4, 0.6.164)

i18n (0.5.0)

json (1.5.1, 1.4.6)

launchy (0.3.7)

less (1.2.21)

libxml-ruby (1.1.2)

liquid (2.2.2)

mail (2.2.19, 2.2.15)

mime-types (1.16)

mocha (0.9.12)

mongrel (1.1.5)

multi_json (0.0.5)

multi_xml (0.2.2)

multipart-post (1.1.0)

mutter (0.5.3)

needle (1.3.0)

net-scp (1.0.1)

net-sftp (2.0.1, 1.1.1)

net-ssh (2.0.4, 1.1.4)

net-ssh-gateway (1.0.0)

nokogiri (1.4.4)

oauth (0.4.5, 0.4.4)

polyglot (0.3.1)

rack (1.2.1, 1.0.1)

rack-mount (0.6.14, 0.6.13)

rack-test (0.5.7)

rails (3.0.9, 3.0.5, 2.3.5, 1.2.6)

railties (3.0.9, 3.0.5)

rake (0.8.7, 0.8.3)

rant (0.5.7)

rash (0.3.0)

rdoc (3.9.2)

RedCloth (4.1.1)

rest-client (1.6.1)

roauth (0.0.3)

rspec (2.6.0, 2.5.0)

rspec-core (2.6.4, 2.5.1)

rspec-expectations (2.6.0, 2.5.0)

rspec-mocks (2.6.0, 2.5.0)

rspec-rails (2.6.1, 2.5.0)

ruby-openid (2.1.2)

ruby-yadis (0.3.4)

rubygems-update (1.6.0)

rubynode (0.1.5)

sequel (3.20.0)

shotgun (0.9)

simple_oauth (0.1.4)

sinatra (1.2.3, 1.0)

spork (0.9.0.rc4)

sqlite3 (1.3.3)

sqlite3-ruby (1.2.4)

stringex (1.2.2)

sys-uname (0.8.5)

taps (0.3.23)

termios (0.9.4)

thor (0.14.6)

tilt (1.2.2)

treetop (1.4.9, 1.4.5)

tweetstream (1.0.4)

twitter (1.4.0)

twitter-stream (0.1.10)

twitter4r (0.7.0)

twitter_oauth (0.4.3)

tzinfo (0.3.24)

uuidtools (2.1.2)

visionmedia-growl (1.0.3)

webrat (0.7.1)

xmpp4r (0.4)

ZenTest (4.5.0)

Answer 1

Uninstalling dm-postgres-adapter 1.2.0.rc2 fixed it.

Answer 2

You should use dm-core instead of datamapper in your require line.

Are you using Bundler? Provided you have bundler set up correctly, it should already be loaded for you.