Thing.first.discussions.started_by(User.first)

discussion.rb

class Discussion < ActiveRecord::Base
  belongs_to :thing

  scope :started_by lambda { |user| where(:initiated_by_id => user) }
end

thing.rb

class Thing < ActiveRecord::Base
  belongs_to :user
  has_many :discussions
end

user.rb

class User < ActiveRecord::Base
  has_many :things
end