In response to: http://rosenfeld.herokuapp.com/en/articles/ruby-rails/2013-12-18-sequel-is-awesome-and-much-better-than-activerecord?utm_source=rubyweekly&utm_medium=email

arel mapped template ids.rb

module Sq
  class Template < Sequel::Model
    one_to_many :fields

    def mapped_template_ids
      FieldMapping.as(:m).
        join(Field.named(:f), id: :field_id, template_id: id).
        join(Field.named(:mf), id: :m__mapped_field_id).
        distinct.select_map(:mf__template_id)
    end
  end
end

# Generates SQL:

#SELECT DISTINCT "mf"."template_id"
#  FROM "field_mappings" AS "m"
#  INNER JOIN "fields" AS "f"
#  ON (
#    ("f"."id" = "m"."field_id") AND ("f"."template_id" = {{id}}))
#  INNER JOIN "fields" AS "mf"
#  ON (
#    "mf"."id" = "m"."mapped_field_id")


module AR
  class Field < ActiveRecord::Base
    belongs_to :template
    belongs_to :parent, class_name: 'Field'
    has_many :children, class_name: 'Field', foreign_key: :parent_id
  end

  class FieldMapping < ActiveRecord::Base
  end

  class Template < ActiveRecord::Base
    has_many :fields

    def mapped_template_ids
      m = FieldMapping.arel_table.alias(:m)
      f = Field.arel_table.alias(:f)
      mf = Field.arel_table.alias(:mf)

      query = FieldMapping.arel_table.
        join(f).on(
          f[:id].eq(m[:field_id]).and(f[:template_id].eq(id))).
        join(mf).on(
          mf[:id].eq(m[:mapped_field_id])).
        project(mf[:template_id]).
        from(m).
        tap {|f| f.distinct }.
        to_sql

      # Generates SQL:
      #
      #  SELECT DISTINCT "mf"."template_id" 
      #  FROM "field_mappings" "m" 
      #  INNER JOIN "fields" "f" 
      #    ON "f"."id" = "m"."field_id" AND "f"."template_id" = {{id}} 
      #  INNER JOIN "fields" "mf" 
      #    ON "mf"."id" = "m"."mapped_field_id"

      ActiveRecord::Base.connection.execute(query).to_a
    end
  end
end

AR::Template.last.mapped_template_ids