message board help

Im trying to do the message board tutorial here but I have rain into a problem

I get this error message

SQLite3::SQLException: table posts already exists: CREATE TABLE posts ("id" INTE GER PRIMARY KEY AUTOINCREMENT NOT NULL, "body" text DEFAULT NULL, "created_at" d atetime NOT NULL, "name" varchar(100) DEFAULT NULL, "discussion_id" integer NOT NULL)

when I try to migrate this bit of code

class CreatePostsTable < ActiveRecord::Migration   def self.up    create_table :posts do |table|     table.column :body, :text     table.column :created_at, :datetime, :null => false     table.column :name, :string, :limit=>100     table.column :discussion_id, :integer, :null => false   end   execute "ALTER TABLE posts ADD CONSTRAINT fk_posts     FOREIGN KEY(discussion_id) REFERENCES discussions(id)"   end   def self.down     drop_table :posts   end end

can anyone help me?