add item set and relation

app/models/item_set.rb

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+class ItemSet < ApplicationRecord
+end

app/models/menu_instance_item_set.rb

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+class MenuInstanceItemSet < ApplicationRecord
+end

app/models/menu_item_set.rb

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+class MenuItemSet < ApplicationRecord
+end

db/migrate/20170811070634_create_item_sets.rb

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+class CreateItemSets < ActiveRecord::Migration[5.1]
+  def change
+    create_table :item_sets do |t|
+      t.string :name
+      t.integer :min_selectable_qty
+      t.integer :max_selectable_qty
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20170811071108_create_menu_item_sets.rb

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+class CreateMenuItemSets < ActiveRecord::Migration[5.1]
+  def change
+    create_table :menu_item_sets do |t|
+      t.references :item_set_id, foreign_key: true
+      t.references :menu_item_id, foreign_key: true
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20170811071327_create_menu_instance_item_sets.rb

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+class CreateMenuInstanceItemSets < ActiveRecord::Migration[5.1]
+  def change
+    create_table :menu_instance_item_sets do |t|
+      t.references :item_set_id, foreign_key: true
+      t.references :menu_item_instance_id, foreign_key: true
+      
+      t.timestamps
+    end
+  end
+end

spec/models/item_set_spec.rb

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+require 'rails_helper'
+
+RSpec.describe ItemSet, type: :model do
+  pending "add some examples to (or delete) #{__FILE__}"
+end

spec/models/menu_instance_item_set_spec.rb

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+require 'rails_helper'
+
+RSpec.describe MenuInstanceItemSet, type: :model do
+  pending "add some examples to (or delete) #{__FILE__}"
+end

spec/models/menu_item_set_spec.rb

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+require 'rails_helper'
+
+RSpec.describe MenuItemSet, type: :model do
+  pending "add some examples to (or delete) #{__FILE__}"
+end