Everybody working with Rails project, slightly more complicated, than To-Do list, is aware of N+1 Query Problem. It is awful, it might drastically decrease the performance of an application. I saw pages, performing over 4000 queries against database on a single load.

There are great diagnostics gems, like aforementioned bullet and/or query_reviewer There are plenty of hints, tips and tricks on how to overcome it (TL;DR: use includes eager loading.)

Unfortunately, there is no clean solution on an opposite problem: multiple updates. Imagine you have to update a table, setting a column value basing on the value in another column.

MySQL (and most other dialects) provides a single query for it:

UPDATE `profiles` SET `yay` = CASE `workflow_state`
                              WHEN 'approved' THEN 'yes'
                              WHEN 'cancelled' THEN 'no'
                              ELSE `yay`

Till now Rails had no nifty wrapper for it. Now it has:

module ActiveRecord
  class Base # :nodoc:
    # Updates multiple rows in table using prepared hash as input
    # @param by_field [String] name of field to be used as `id`
    # @param prepared_updates [Hash] batched update, hash consisting of
    #        field_names ⇒ hash of maps {id ⇒ value}
    # The query to be prepared and executed:
    #     UPDATE `profiles` SET `yay` = CASE `workflow_state`
    #     WHEN 'approved' THEN 'yes'
    #     WHEN 'cancelled' THEN 'no'
    #     ELSE `yay`
    #     END
    # Code for that:
    #     update_multiple 'id', { yay: { approved: :yes, cancelled: 'no' } }
    # Real life example:
    #     Profile.update_multiple :id,  {
    #           address: { 9 => 'Avda Success', 287 => 'Avda Failure' },
    #           province: { 9 => 'Siberia', 287 => 'Siberia II' } }
    def self.update_multiple by_field, prepared_updates
      query = [
        "UPDATE `#{self.table_name}`",
        prepared_updates.map do |field, prepared_update|
         "`#{field}` = #{__case_block_of_multiple_rows_update_query by_field, field, prepared_update}"
      ].join $/
      ActiveRecord::Base.connection.execute query

    def self.__case_block_of_multiple_rows_update_query by_field, field, prepared_update
        "CASE `#{by_field}`",
        prepared_update.map do |id, val|
          val = "'#{val}'" unless val.is_a?(Numeric)  # Date??
          "\tWHEN '#{id}' THEN #{val}"
        "\tELSE `#{field}`",
      ].join $/

The above code will update a table with single query, according to the hash given. Enjoy!