basic support for PostgreSQL Enums in ActiveRecord - to be placed in an initializer

pg_enum.rb

##
# This provides `create_enum` and `drop_enum` methods for migrations, which creates
# a Postgres ENUM type with the provided values.
#
# It'll also dump these into the schema.rb to be loaded into another DB (e.g. rake db:test:load)
#
# In order to reference the new enums as actual types, ActiveRecord needs to know about them, so
# make sure they are all represented in the `types` array below
#
# Then you can reference them in your migrations, e.g.
# 
# create_table :example do
#   t.state :state
#   t.location :location
#   t.method :method
# end
##

types = %w(state location method)

module PgEnum
  
  # provide the enum DDL methods
  module PostgreSQLAdapter
    def create_enum(name, values)
      values = values.map {|v| "'#{v}'"}
      execute "CREATE TYPE #{name} AS ENUM (#{values.join(', ')})"
    end

    def drop_enum(name)
      execute "DROP TYPE #{name}"
    end
  end

  # provide support for writing out the 'create_enum' calls in schema.rb
  module SchemaDumper
    extend ActiveSupport::Concern

    included do
      alias_method_chain :tables, :enums
    end

    def tables_with_enums(stream)
      enums(stream)
      tables_without_enums(stream)
    end

    private
    def enums(stream)
      enums = @connection.select_all("SELECT typname FROM pg_type WHERE typtype = 'e'").map{|r| r['typname']}

      statements = []

      enums.each do |enum|
        values = @connection.select_all(
          "SELECT e.enumlabel FROM pg_enum e JOIN pg_type t ON e.enumtypid = t.oid WHERE t.typname = '#{enum}'"
        ).map {|v| v['enumlabel'].inspect }.join(', ')

        statements << "  create_enum(#{enum.inspect}, [#{values}])"
      end

      stream.puts statements.join("\n")
      stream.puts
    end
  end
end


class ActiveRecord::ConnectionAdapters::PostgreSQLAdapter
  include PgEnum::PostgreSQLAdapter
end

module ActiveRecord::ConnectionAdapters
  include PgEnum::PostgreSQLAdapter
end

class ActiveRecord::SchemaDumper
  include PgEnum::SchemaDumper
end

# make activerecord aware of our enum types

types.each do |type|
  ActiveRecord::ConnectionAdapters::PostgreSQLAdapter::NATIVE_DATABASE_TYPES[type.to_sym] = { name: type }

  ActiveRecord::ConnectionAdapters::Column.class_eval do
    
    define_method("simplified_type_with_#{type}") do |field_type|
      if field_type == type
        field_type.to_sym
      else
        send("simplified_type_without_#{type}", field_type)
      end
    end

    alias_method_chain :simplified_type, type
  end

  ActiveRecord::ConnectionAdapters::PostgreSQLAdapter::TableDefinition.class_eval do

    define_method(type) do |*args|
      options = args.extract_options!
      column(args[0], type, options)
    end
  end

  ActiveRecord::ConnectionAdapters::PostgreSQLAdapter::OID.alias_type type, 'text'
  
  ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.tap do |clazz|
    clazz::OID.register_type(type, clazz::OID::Identity.new)
  end
end

Have a look at this @eliduke,

https://gist.github.com/Envek/7077bfc36b17233f60ad

I had the same issue regarding the simplified_type method absense. i've perfectioned the implementation for rails 4.2 and improved support for schema.rb when dumping tables with pg enum columns. I hope this saves someone time:
https://gist.github.com/khamusa/77ada9895db15dbee4e57cb715830a48