ruby on rails rake db:migrate error

i'm starting to learn ruby on rails using this guide : getting_started , i created my project and database but when i run rake db:migrate i get this error:

@mona-Extensa-5230:~/rubyDev/Sites/blog# rake db:migrate

(in /home/mona/rubyDev/Sites/blog)

== CreatePosts: migrating ==================开发者_StackOverflow中文版================================== -- create_table(:posts)

rake aborted!

An error has occurred, this and all later migrations canceled:

private method `String' called for#

ActiveRecord::ConnectionAdapters::TableDefinition:0xb7540f30>

thanks.

Looks like you have a line like this in one of your migrations:

t.String

Note that the s needs to be in lowercase (t.string)