Option A

i = Message::Identifier.new( hl7_file )

def init_patient
  self.patient = find_patient(i.patient_criteria)
  some error  if( patient.blank? )
end    

def init_visit
  self.visit = find_visit( i.visit_criteria( patient ))
  some_error if ( self.visit.blank? )
end

def init_order
  self.order = find_order( self.visit )
  some_error if self.order.blank?
end

In this case, we find the patient. If patient exists, find visit. If visit exists, find order.

Option B

i = Message::Identifier.new( hl7_file )

def init_order
  self.order = find_order( i.all_criteria )

  if( self.order.blank? )
    patient = find_patient( i.patient_criteria )
    some_error if self.patient.blank?

    if( self.patient )
      visit = find_visit( i.visit_criteria )
      some_error if self.visit.blank?
    end

    if( self.visit )
      error "couldn't find an order for visit #{visit.id}"
    end
  end
end
    

in this case, we do one big query. If we don't find the order, then we retrace our steps in order to figure out where the issue happened.