DEFAULT literals in AR for postgres defaults (via @copiousfreetime)

ar_literals.rb

# Force ActiveRecord to insert the 'DEFAULT' literal for columns which
# are non-null but do not have a default that AR understands.
#
# MySQL is lenient in that if you insert a NULL in something with a default,
# the database will just insert the default anyway. Postgres is not so kind:
# it'll do exactly what you told it, causing an error.
# Usually AR can figure out the default for a column, but if it's a default
# value that the connection adapter can't figure out, it'll insert a NULL.
# So: allow explicit 'DEFAULT' literals (which is SQL standard) to be used instead
# where appropriate.
#

require 'set'

# This exists as a way to make active record use SQL literals in attributes when
# creating or updating records
module ActiveRecord
  module ConnectionAdapters
    class Column
      class Literal < ::String
        def quoted_id() self end
      end

      def type_cast_with_literal(value)
        if Literal === value
          value
        else
          type_cast_without_literal(value)
        end
      end
      alias_method_chain :type_cast, :literal

    end
  end

  class Base
    def self.Literal(*args, &block)
      ::ActiveRecord::ConnectionAdapters::Column::Literal.new(*args, &block)
    end
  end

  def self.Literal(*args, &block)
    Base.Literal(*args, &block)
  end
end

module ActiveRecord
  class Base
    cattr_accessor :db_default_columns

    # force these columns to use DEFAULT on create
    def self.use_db_default_in_columns(*column_names)
      self.db_default_columns ||= Set.new
      column_names.each { |c| self.db_default_columns << c }
    end

    before_create :force_default_where_necessary
    after_save :reload_if_default_forced

    # force the sql keyword DEFAULT into those columns where there must be a
    # default value, that is, columns with a default that are not nullable, those
    # should have the DEFAULT keyword set
    def force_default_where_necessary
      @default_forced = false
      @attributes.each_pair do |name, value|
        unless value
          column = column_for_attribute(name)
          if self.class.db_default_columns.include?(column.name) || (column.has_default? && !column.null)
            write_attribute name, ActiveRecord::Literal('DEFAULT')
            @default_forced = true
          end
        end
      end
    end

    # if a default was forced, then we have to load the value that was stored in
    # the db into the record, AR won't, it'll just leave it as 'DEFAULT' which
    # really won't help much.
    #
    def reload_if_default_forced
      if defined?( @default_forced ) && @default_forced then
        # force a save of targets ?
        self.reload
        @default_forced = false
      end
    end
  end
end