class Foo < ActiveRecord::Base
  has_one :bar, ->(current_user) { where(user_id:current_user) }
end

class Bar < ActiveRecord::Base
  belongs_to :foo
  belongs_to :user
end


class User < ActiveRecord::Base
  has_one :bar
end

> Foo.first.bar(2)
SQL  =>  Foo Load (1.2ms)  SELECT  "foos".* FROM "foos"  ORDER BY "foos"."id" ASC LIMIT 1
  Bar Load (0.7ms)  SELECT  "bars".* FROM "bars" WHERE "bars"."foo_id" = ?
AND "bars"."user_id" = 1 LIMIT 1  [["foo_id", 1]]