Compressed fields in Mongoid 6.4+

compressed_hash.rb

require 'zstd-ruby'

class CompressedHash < Hash
  DICTIONARY = IO.read('config/dictionaries/1')
  def mongoize
    if size > 0 # only compress non-empty hash
      # BSON::Binary.new(Zstd.compress(self.to_bson.to_s))
      BSON::Binary.new(Zstd.compress_using_dict(self.to_bson.to_s, DICTIONARY))
    else
      self
    end
  end

  class << self
    # Get the object as it was stored in the database, and instantiate
    # this custom class from it.
    def demongoize(object)
      case object
      # when BSON::Binary then CompressedHash[BSON::Document.from_bson(BSON::ByteBuffer.new(Zstd.decompress(object.data)))]
      when BSON::Binary then CompressedHash[BSON::Document.from_bson(BSON::ByteBuffer.new(Zstd.decompress_using_dict(object.data, DICTIONARY)))]
      when Hash then CompressedHash[object]
      else object
      end
    end

    # Takes any possible object and converts it to how it would be
    # stored in the database.
    def mongoize(object)
      case object
      when CompressedHash then object.mongoize
      when Hash then CompressedHash[object].mongoize
      else object
      end
    end

    # Converts the object that was supplied to a criteria and converts it
    # into a database friendly form.
    alias_method :evolve, :mongoize
  end
end

compressed_string.rb

require 'zlib'

class CompressedString < String
  def mongoize
    if bytesize > 1000 # only compress over 1k
      BSON::Binary.new(Zlib::Deflate.deflate(self))
    else
      self
    end
  end

  class << self
    # Get the object as it was stored in the database, and instantiate
    # this custom class from it.
    def demongoize(object)
      case object
      when BSON::Binary then self.new(Zlib::Inflate.inflate(object.data))
      when String then self.new(object)
      else object
      end
    end

    # Takes any possible object and converts it to how it would be
    # stored in the database.
    def mongoize(object)
      case object
      when self then object.mongoize
      when String then self.new(object).mongoize
      else object
      end
    end

    # Converts the object that was supplied to a criteria and converts it
    # into a database friendly form.
    alias_method :evolve, :mongoize
  end
end

# Decompress-only class to support removing compression smoothly
class UnCompressedString < CompressedString
  def mongoize
    self
  end
end