Creating a table categorization with has_many :through for categories and posts

Hey guys I'm doing a has_many :through association in ruby on rails. And I want some help if it's ok with my example. So I pretend to do categories for Posts. Supposing I already have builded the class post. So for create the database for categories I pretend to do like these:

class CreateCategories < ActiveRecord::Migration[5.0]
  def change
    create_table :categories do |t|
      t.string :name
      t.timestamps
    end
  end

  def change
    create_table :categorizations do |t|
      t.belongs_to :post, index: true
      t.belongs_to :category, index: true
      t.integer :position

      t.timestamps
    end

    add_index :categorizations, [:product_id, :category_id], unique: true
  end
end

It's ok these index's for boost the database?

Yes, that multi-column index that you've defined at the bottom:

add_index :categorizations, [:product_id, :category_id], unique: true

is valid. But it's also unnecessary and likely less performant than the single column indexes that you have individually defined on product_id and category_id.