h_m_t

class Good < ActiveRecord::Base
  has_many :good_tags
  has_many :tags, through: :good_tags
end

class GoodTag < ActiveRecord::Base
  belongs_to :good
  belongs_to :tag
end

class Tag < ActiveRecord::Base
  has_many :good_tags
  has_many :goods, through: :good_tags
end

2.1.1 :002 > g = Good.create
   (0.2ms)  begin transaction
  SQL (0.7ms)  INSERT INTO "goods" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2015-11-15 17:35:13.480173"], ["updated_at", "2015-11-15 17:35:13.480173"]]
   (0.6ms)  commit transaction
 => #<Good id: 1, created_at: "2015-11-15 17:35:13", updated_at: "2015-11-15 17:35:13"> 
2.1.1 :003 > g.tags.create
   (0.0ms)  begin transaction
  SQL (0.5ms)  INSERT INTO "tags" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2015-11-15 17:35:23.303590"], ["updated_at", "2015-11-15 17:35:23.303590"]]
  SQL (0.2ms)  INSERT INTO "good_tags" ("good_id", "tag_id", "created_at", "updated_at") VALUES (?, ?, ?, ?)  [["good_id", 1], ["tag_id", 1], ["created_at", "2015-11-15 17:35:23.321685"], ["updated_at", "2015-11-15 17:35:23.321685"]]
   (0.8ms)  commit transaction
 => #<Tag id: 1, created_at: "2015-11-15 17:35:23", updated_at: "2015-11-15 17:35:23"> 
2.1.1 :004 > g.tags.create
   (0.1ms)  begin transaction
  SQL (1.8ms)  INSERT INTO "tags" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2015-11-15 17:35:34.638708"], ["updated_at", "2015-11-15 17:35:34.638708"]]
  SQL (0.2ms)  INSERT INTO "good_tags" ("good_id", "tag_id", "created_at", "updated_at") VALUES (?, ?, ?, ?)  [["good_id", 1], ["tag_id", 2], ["created_at", "2015-11-15 17:35:34.643055"], ["updated_at", "2015-11-15 17:35:34.643055"]]
   (1.7ms)  commit transaction
 => #<Tag id: 2, created_at: "2015-11-15 17:35:34", updated_at: "2015-11-15 17:35:34"> 
2.1.1 :005 > g.tags.create
   (0.1ms)  begin transaction
  SQL (1.3ms)  INSERT INTO "tags" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2015-11-15 17:35:35.425569"], ["updated_at", "2015-11-15 17:35:35.425569"]]
  SQL (0.2ms)  INSERT INTO "good_tags" ("good_id", "tag_id", "created_at", "updated_at") VALUES (?, ?, ?, ?)  [["good_id", 1], ["tag_id", 3], ["created_at", "2015-11-15 17:35:35.429225"], ["updated_at", "2015-11-15 17:35:35.429225"]]
   (1.6ms)  commit transaction
 => #<Tag id: 3, created_at: "2015-11-15 17:35:35", updated_at: "2015-11-15 17:35:35"> 
2.1.1 :006 > Good.last.tags
  Good Load (0.2ms)  SELECT  "goods".* FROM "goods"  ORDER BY "goods"."id" DESC LIMIT 1
  Tag Load (0.2ms)  SELECT "tags".* FROM "tags" INNER JOIN "good_tags" ON "tags"."id" = "good_tags"."tag_id" WHERE "good_tags"."good_id" = ?  [["good_id", 1]]
 => #<ActiveRecord::Associations::CollectionProxy [#<Tag id: 1, created_at: "2015-11-15 17:35:23", updated_at: "2015-11-15 17:35:23">, #<Tag id: 2, created_at: "2015-11-15 17:35:34", updated_at: "2015-11-15 17:35:34">, #<Tag id: 3, created_at: "2015-11-15 17:35:35", updated_at: "2015-11-15 17:35:35">]> 
 
   create_table "good_tags", force: :cascade do |t|
    t.integer  "good_id"
    t.integer  "tag_id"
    t.string   "othervalue"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  create_table "goods", force: :cascade do |t|
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  create_table "tags", force: :cascade do |t|
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end