class ActiveRecord::Associations::CollectionAssociation

def ids_writer(ids)

Implements the ids writer method, e.g. foo.item_ids= for Foo.has_many :items 
def ids_writer(ids)
  primary_key = reflection.association_primary_key
  pk_type = klass.type_for_attribute(primary_key)
  ids = Array(ids).compact_blank
  ids.map! { |i| pk_type.cast(i) }
  records = klass.where(primary_key => ids).index_by do |r|
    r.public_send(primary_key)
  end.values_at(*ids).compact
  if records.size != ids.size
    found_ids = records.map { |record| record.public_send(primary_key) }
    not_found_ids = ids - found_ids
    klass.all.raise_record_not_found_exception!(ids, records.size, ids.size, primary_key, not_found_ids)
  else
    replace(records)
  end
end