seed.rb

admin = Role.create(:title => "admin")
mike  = User.create(:name => "mike")

# either
mike.roles << admin
# or
mike.roles.push admin

# or you can also go the other way
admin.users << mike
# or
admin.users.push mike


# a quick way to create a bunch of roles
['admin', 'worker', 'client'].each do |title|
  Role.create(:title => title)
end