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MySQL 5.7 Reference - https://dev.mysql.com/doc/refman/5.7/en/create-table-secondary-indexes.html#json-column-indirect-index
Source - https://github.com/driftingruby/088-virtual-columns-with-json-data-types
Episode #87 Virtual Columns in MySQL - https://www.driftingruby.com/episodes/virtual-columns-in-mysql

Summary

Terminalrails g scaffold product name properties:json color:virtual size_id:virtual:index

db/migrate/20170707050032_create_products.rbclass CreateProducts < ActiveRecord::Migration[5.1]
  def change
    create_table :products do |t|
      t.string :name
      t.json :properties
      t.virtual :color, type: :string, as: "JSON_UNQUOTE(JSON_EXTRACT(properties, '$.color'))"
      t.virtual :size_id, type: :integer, as: "JSON_UNQUOTE(JSON_EXTRACT(properties, '$.size_id'))", stored: true
      t.timestamps
    end
    add_index :products, :size_id
  end
end

products/_form.html.erb  <%= form.fields_for :properties do |builder| %>
    <%= content_tag :div, class: :field do %>
      <%= builder.label :color, label: 'Color' %>
      <%= builder.text_field :color, value: product.color %>
    <% end %>

    <%= content_tag :div, class: :field do %>
      <%= builder.label :size_id, label: 'Size' %>
      <%= builder.collection_select :size_id, Size.all, :id, :name, selected: product.size_id %>
    <% end %>
  <% end %>

products_controller.rb    def product_params
      params.require(:product).permit(:name, properties: [:color, :size_id])
    end

products/show.html.erb<%= @product.color %>

<%= @product.size&.name %>

product.rbclass Product < ApplicationRecord
  belongs_to :size, optional: true
end

size.rbclass Size < ApplicationRecord
  has_many :products
end