Este error es muy comun cuando empezamos a trabajar con Rails, y no te nemos grandes conocimientos sobre el tema, al momento que nosotros creamos un proyecto en rails con el comando

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CODE:
  1. rails my_proyect
  2. create
  3. create  app/controllers
  4. create  app/helpers
  5. create  app/models
  6. create  app/views/layouts
  7. create  config/environments
  8. create  config/initializers
  9. create  db
  10. create  doc
  11. create  lib
  12. create  lib/tasks
  13. create  log
  14. create  public/images
  15. create  public/javascripts
  16. create  public/stylesheets
  17. create  script/performance
  18. create  script/process
  19. create  test/fixtures
  20. create  test/functional
  21. create  test/integration
  22. create  test/unit
  23. create  vendor
  24. create  vendor/plugins
  25. create  tmp/sessions
  26. create  tmp/sockets
  27. create  tmp/cache
  28. create  tmp/pids
  29. create  Rakefile
  30. create  README
  31. create  app/controllers/application.rb
  32. create  app/helpers/application_helper.rb
  33. create  test/test_helper.rb
  34. create  config/database.yml
  35. create  config/routes.rb
  36. create  config/initializers/inflections.rb
  37. create  config/initializers/mime_types.rb
  38. create  config/initializers/new_rails_defaults.rb
  39. create  config/boot.rb
  40. create  config/environment.rb
  41. create  config/environments/production.rb
  42. create  config/environments/development.rb
  43. create  config/environments/test.rb
  44. create  script/about
  45. create  script/console
  46. create  script/dbconsole
  47. create  script/destroy
  48. create  script/generate
  49. create  script/performance/benchmarker
  50. create  script/performance/profiler
  51. create  script/performance/request
  52. create  script/process/reaper
  53. create  script/process/spawner
  54. create  script/process/inspector
  55. create  script/runner
  56. create  script/server
  57. create  script/plugin
  58. create  public/dispatch.rb
  59. create  public/dispatch.cgi
  60. create  public/dispatch.fcgi
  61. create  public/404.html
  62. create  public/422.html
  63. create  public/500.html
  64. create  public/index.html
  65. create  public/favicon.ico
  66. create  public/robots.txt
  67. create  public/images/rails.png
  68. create  public/javascripts/prototype.js
  69. create  public/javascripts/effects.js
  70. create  public/javascripts/dragdrop.js
  71. create  public/javascripts/controls.js
  72. create  public/javascripts/application.js
  73. create  doc/README_FOR_APP
  74. create  log/server.log
  75. create  log/production.log
  76. create  log/development.log
  77. create  log/test.log

Nos crea por default en la carpeta config de nuestro proyecto un archivo llamado database.yml el cual contiene algo parecido

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CODE:
  1. # SQLite version 3.x
  2. #   gem install sqlite3-ruby (not necessary on OS X Leopard)
  3. development:
  4. adapter: sqlite3
  5. database: db/development.sqlite3
  6. timeout: 5000
  7.  
  8. # Warning: The database defined as "test" will be erased and
  9. # re-generated from your development database when you run "rake".
  10. # Do not set this db to the same as development or production.
  11. test:
  12. adapter: sqlite3
  13. database: db/test.sqlite3
  14. timeout: 5000
  15.  
  16. production:
  17. adapter: sqlite3
  18. database: db/production.sqlite3
  19. timeout: 5000

Este archivo nos indica la configuracion que tomara Rails para acceder a la base de datos, podemos observar que nos hacen la indicacion de que hay que instalar sqlite3-ruby esto lo hacemos con el comando.

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CODE:
  1. gem install sqlite3-ruby

Pero si nosotros usamos otra base de datos por ejemplo en mysql, hay que cambiar la configuracion, para poder acceder a mysql y no a sqlite.

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CODE:
  1. development:
  2. adapter: mysql
  3. database: recetario
  4. username: root
  5. password: sigmaboy
  6. host: localhost
  7.  
  8. # Warning: The database defined as "test" will be erased and
  9. # re-generated from your development database when you run "rake".
  10. # Do not set this db to the same as development or production.
  11. test:
  12. adapter: mysql
  13. database: recetario
  14. username: root
  15. password: sigmaboy
  16. host: localhost
  17.  
  18. production:
  19. adapter: mysql
  20. database: recetario
  21. username: root
  22. password: sigmaboy
  23. host: localhost

Con esto podemos solucionar el error de

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CODE:
  1. no such file to load -- sqlite3

:D