packed_column.rb

# In a couple of tables we have VARBINARY columns which either encode a packed
# integer of some kind, or a checksum (which is commonly shared as a hexadecimal).
# This module allows rapid definition of such columns and accessors for them. For example,
# imagine you want to store a CRC32 value - which fits into a uint4
#
#   class RecordWithChecksum < ActiveRecord::Base
#     extend PackedColumn
#     packed_value_column :crc32, pattern: 'V'
#   end
#
#   # This will set the `crc32` column to the encoded value:
#   record.crc32 = Zlib.crc32(some_payload)
#
# The column you create must be BLOB, BINARY or VARBINARY type.
module PackedColumn
  class SingleValuePackCoder < Struct.new(:pattern)
    def dump(value)
      value ? [value].pack(pattern) : nil
    end

    def load(bin_str)
      bin_str ? bin_str.unpack(pattern).first : nil
    end
  end

  def packed_value_column(column, pattern:)
    serialize column.to_s, SingleValuePackCoder.new(pattern.to_s)
  end
end