module ActiveRecord::Relation::RecordFetchWarning

# File activerecord/lib/active_record/relation/record_fetch_warning.rb, line 16
def exec_queries
  QueryRegistry.reset

  super.tap do
    if logger && warn_on_records_fetched_greater_than
      if @records.length > warn_on_records_fetched_greater_than
        logger.warn "Query fetched #{@records.size} #{@klass} records: #{QueryRegistry.queries.join(";")}"
      end
    end
  end
end

When this module is prepended to ActiveRecord::Relation and config.active_record.warn_on_records_fetched_greater_than is set to an integer, if the number of records a query returns is greater than the value of warn_on_records_fetched_greater_than, a warning is logged. This allows for the detection of queries that return a large number of records, which could cause memory bloat.

In most cases, fetching large number of records can be performed efficiently using the ActiveRecord::Batches methods. See ActiveRecord::Batches for more information.