rails method for transaction type
It looks like row is simply a hash, but it has various value combinations that are significant.

You could refactor the detection of those combinations, for example by creating a class based on the row that has an attribute for #transaction_type ...

class Row
  def initialize(row)
    @row = row
  end

  def sender_blz
    row['SENDER_BLZ']
  end

  def receiver_blz
    row['RECEIVER_BLZ']
  end

  def umsatz_key
    row['UMSATZ_KEY']
  end

  def transaction_type
    return "AccountTransfer" if sender_blz   == "00000000" &amp;&amp; receiver_blz == "00000000"
    return "BankTransfer"    if sender_blz   == "00000000" &amp;&amp; umsatz_key == "10"
    return "Lastschrift"     if receiver_blz == "70022200" &amp;&amp; umsatz_key == "16"
    false
  end
end


Then your method becomes

Row.new(row).transaction_type


I would rather see magic strings like "00000000" replaced with constants, and conditions like sender_blz == "00000000" replaced with methods that give a name to the condition, like:

def sender_not_specified?
  sender_blz == NOT_SPECIFIED
end


... thus you would have something like ...

  def transaction_type
    return "AccountTransfer" if sender_not_specified? &amp;&amp; receiver_not_specified?
    return "BankTransfer"    if sender_not_specified? &amp;&amp; something_about_umsatz?
    return "Lastschrift"     if receiver_not_specified? &amp;&amp; something_else_about_umsatz?
    false
  end