Migration to fix serialized attributes on Versions made with paper_trail 4.0.2 or earlier

patch_versions.rb

class PatchVersions < ActiveRecord::Migration

  # This modifies PaperTrail versions made with 4.0.2 or before to be compatible with 4.1.0.
  #
  # The difference is that 4.0.2 stored serialized attributes in their native (reified) form,
  # while 4.1.0 and above store serialized attributes in their serialized form; e.g.:
  #   {'foo'=>52, 'bar'=>'baz'}  <----- 4.0.2 yamlizes this Hash
  #   '{"foo":52,"bar":"baz"}'   <----- 4.1.0 yamlizes this String
  # This causes 4.1.0 to raise errors when attempting to reify versions created with 4.0.2.
  #
  def up

    # Get the list of all extant model classes currently referenced by PaperTrail versions:
    models = PaperTrail::Version.pluck(:item_type).uniq.sort.map do |k|
      begin
        k.constantize
      rescue NameError
        nil
      end
    end.compact

    # Determine which ones have serialized attributes:
    serialized = Hash[ models.zip( models.map{ |m| m.columns.select{ |c| ActiveRecord::Type::Serialized === c.cast_type } } ) ]
    serialized.delete_if{ |_,c| c.blank? }

    # Perform all fixes in a single transaction so no data is lost if the migration fails:
    PaperTrail::Version.transaction do
      serialized.each do |klass,cols|

        # For each affected model, correct each affected Version record:
        print "Patching #{klass.name} versions (#{cols.map(&:name).join(', ')})"
        PaperTrail::Version.where( item_type: klass.name ).each do |v|
          obj  = v.object         && YAML.load( v.object )
          chgs = v.object_changes && YAML.load( v.object_changes )

          cols.each do |c|
            # For each serialized attribute, if this version contains that attribute, replace it with its serialized form:
            obj[  c.name ] = c.cast_type.coder.dump( obj[ c.name ] )  if obj  &&  obj[ c.name ]
            chgs[ c.name ].map!{ |x| x && c.cast_type.coder.dump(x) } if chgs && chgs[ c.name ]
          end

          # Replace the YAML strings on the Version record with the corrected strings:
          updates = {}
          updates[:object]         = YAML.dump( obj )  if obj
          updates[:object_changes] = YAML.dump( chgs ) if chgs
          v.update! updates if updates.present?

          print '.'
        end
        puts

      end
    end

  end

  def down
    raise ActiveRecord::IrreversibleMigration # In principle this could be done, but has not been implemented.
  end

end