Alguma maneira de concatenar isso:

#<ActiveRecord::Relation [#<Activity::AttachDocument id: 1, archive: "ata.pdf", number: "", status: 9502, created_at: "2015-06-01 13:46:10", updated_at: "2015-06-01 13:46:10", progress_id: 3, procedure_id: 2, selected: false>]>

com isso:

#<Activity::AttachDocument id: 3, archive: "pet.pdf", number: "", status: 9500, created_at: "2015-06-01 13:46:25", updated_at: "2015-06-01 13:46:25", progress_id: 5, procedure_id: 2, selected: false>