gistfile1.rb

require "dm-mapper"
require "data_mapper/engine/arel"

DM = DataMapper::Environment.new
DM.engines[:arel] = DataMapper::Engine::Arel::Engine.new("sqlite3:/db/development.sqlite3")

require "virtus"

class Variant
  include DataMapper::Model

  attribute :id, Integer, key: true
end

DM.build(Variant, :arel) do
  key :id
end

DM.finalize

DM[Variant].to_a
# EXCEPTION: => undefined method `to_sym' for 0:Fixnum

gistfile2.rb

# Debugging using debugger

(rdb:1) p relation.class
DataMapper::Engine::Arel::Gateway

(rdb:1) p relation.send(:read)
[{"id"=>1, 0=>1}, {"id"=>2, 0=>2}, {"id"=>3, 0=>3}]

(rdb:1) p relation.send(:to_sql)
"SELECT id FROM \"variants\" "

(rdb:1) p relation.send(:connection).execute("select id from variants")
[{"id"=>1, 0=>1}, {"id"=>2, 0=>2}, {"id"=>3, 0=>3}]