You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: merge.rb
    path of file: ./repos/occams-record/lib/occams-record
    
    the code of the file until where you have to start completion: module OccamsRecord
  #
  # Represents a merge operation to be performed. Merges are always "left" merges. You initialize the
  # Merge with the "left" records, and the name of the attribute into which "right" records will be placed.
  #
  # After initializing, perform a specific type of merge by 
    
    def single!(assoc_rows, mapping)
      target_attrs = mapping.keys
      assoc_attrs = mapping.values

      # Optimized for merges where there's a single mapping key pair (which is the vast majority)
      if mapping.size == 1
        target_attr, assoc_attr = target_attrs[0], assoc_attrs[0]
        assoc_rows_by_ids = assoc_rows.each_with_object({}) { |assoc_row, acc|
          begin
            id = assoc_row.send assoc_attr
          rescue NoMethodError => e
            raise MissingColumnError.new(assoc_row, e.name)
          end