ActiveRecord each_by_page

duncanbeevers (Duncan Beevers) — Tue, 20 Feb 2007 23:59:50 GMT

Perform an operation on a set of models including ActiveRecord callbacks, without instantiating all models at once.

Step through and instantiate each member of the class and execute on it, but instantiate no more than per_page instances at any given time.
Safe for destructive actions or actions that modify the fields your :order or :conditions clauses operate on.



module ActiveRecord

  class Base

    def each_by_page per_page, options = {}, &block

      # By-id for model-modifying blocks

      # Build SQL to get ids of all matching records using the options provided by the user

      sql = construct_finder_sql(options.dup.merge({ :select => 'id' }))

      # Get the results as an array of tiny hashes { "id" => "1" } and flatten them out to just the ids

      all_ids = connection.select_all(sql).map { |h| h['id'] }

      at_a_time = 0..(per_page-1)

 

      # chop apart the all_ids array a segment at a time

      begin

        ids = all_ids.slice!(at_a_time)

        ids_cases = []

        ids.each_with_index { |id, i| ids_cases << "WHEN #{id} THEN #{i}" }

        ids_cases = ids_cases.join(' ')

 

        # Do the deed on this page of results

        find(:all, options.merge(

          :conditions => [ 'id IN (?)', ids ],

          :order => "CASE id #{ids_cases} END"

        )).each &block

 

      end until all_ids.empty?

    end

  end

end

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ActiveRecord each_by_page