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: consistency_checker.rb
    path of file: ./repos/gitlabhq/lib/gitlab/database
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module Gitlab
  module Database
    class ConsistencyChecker
      BATCH_SIZE = 500
      MAX_BATCHES = 20
      MAX_RUNTIME = 5.seconds # must be less than the scheduling frequency of the ConsistencyCheck jobs

      delegate :monotonic_time, to: :'Gitlab::Metrics::System'

      def initialize(source_model:, target_model:, source_columns:, target_columns:)
        @source_model = source_model
        @target_model = target_model
        @source_columns = source_columns
        @target_columns = target_columns
        @source_sort_column = source_columns.first
        @target_sort_column = target_columns.first
        @result = { matches: 0, mismatches: 0, batches: 0, mismatches_details: [] }
      end

      # rubocop:disable Metrics/AbcSize
      def execute(start_id:)
        current_start_id = start_id

   
    
      def append_mismatches_details(source_data, target_data)
        # Mapping difference the sort key to the item values
        # source - target
        source_diff_hash = (source_data - target_data).index_by { |item| item.shift }
        # target - source
        target_diff_hash = (target_data - source_data).index_by { |item| item.shift }

        matches = source_data.length - source_diff_hash.length

        # Items that exist in the first table + Different items
        source_diff_hash.each do |id, values|
          result[:mismatches_details] << {
            id: id,
            source_table: values,
            target_table: target_diff_hash[id]
          }
        end

        # Only the items that exist in the target table
        target_diff_hash.each do |id, values|
          next if source_diff_hash[id] # It's already added

          result[:mismatches_details] << {
            id: id,
            source_table: source_diff_hash[id],
            target_table: values
          }
        end

        matches
      end