Left outer joining 2 fields referring to same User model in Rails 3

arel_joins.rb

t = Task.arel_table
u = User.arel_table
c = u.alias("creators")

keyword = "%#{params}%"

relation = joins("LEFT OUTER JOIN users ON users.id = tasks.user_assigned_id")
.joins("LEFT OUTER JOIN users as #{c.name} ON #{c.name}.id = tasks.creator_id")
.where(
 t[:id].matches(keyword)
 .or(u[:username].matches(keyword))
 .or(c[:username].matches(keyword))
)

Some background information:

class Task < ActiveRecord::Base
belongs_to :user_assigned, :class_name => "User", :foreign_key => :user_assigned_id
belongs_to :creator, :class_name => "User", :foreign_key => :creator_id
end

Intention:
Wanted to search for username belong to either the user_assigned or creator