Ruby ActiveRecord importing library
Notes:


Your code looks pretty good, but I personally wouldn't have written it as a mixable module, this code seemps completely independent from ActiveRecord. I'd write a SpreadSheetReader class and just use it from wherever you want (this way the code is more modular).
Where is import_from called from? it seems some code is missing.
This vat_in conditional seems too verbose. Which are the possible values of row[:client]?


I'd write something like:

company = company_from_somewhere_i_dont_know
columns = [:name, :client, &lt;other columns&gt;...]

SpreadSheetReader.rows(:columns =&gt; columns).map do |row|
  invoice = company.invoices.find_or_initialize_by_name(row[:name].to_s)
  vat_in = row[:client].is_a?(String) ? row[:client] : row[:client].to_i.to_s
  client = company.clients.find_by_vat_in!(vat_in)
  invoice.update_attributes!({
    :pay_date =&gt; row[:pay_date],
    :full_amount =&gt; row[:full_amount].to_i,
    :paid_amount =&gt; row[:paid_amount].to_i,
  })
  invoice
end