https://github.com/dockyard/postgres_ext Integer Array save with String value.

Gemfile.lock

rails (3.2.13)
pg (0.14.1)
pg_array_parser (0.0.8)
postgres_ext (0.3.1)
  activerecord (~> 3.2.0)
  pg_array_parser (~> 0.0.8)

migration.rb

class AddSeenByToDocuments < ActiveRecord::Migration
  def change
    add_column :documents, :seen_by, :integer, array: true, default: []
  end
end

test.rb

[19] pry(main)> d.seen_by = [4, "f"]
=> [4, "f"]
[20] pry(main)> d.save
    ...
    (1.2ms)  UPDATE "documents" SET "seen_by" = '{4,0}' ...
    ...
=> true
[21] pry(main)> d.reload.seen_by
  Document Load (0.6ms)  SELECT "documents".* FROM "documents" WHERE "documents"."id" = $1 LIMIT 1  [["id", 108]]
=> [4, 0]