Why Are There Two Queries Run When Using Includes in a has_one relationship

Question

class Product < ActiveRecord::Base
  attr_accessible :brand_id, :name

  belongs_to :brand
end

class Brand < ActiveRecord::Base
  attr_accessible :name

  has_many :products

end

Controller Action:

  def index
    @products = Product.includes(:brand)

    respond_to do |format|
      format.html # index.html.erb
      format.json { render json: @products }
    end
  end

And View:

<% @products.each do |product| %>
  <tr>
    <td><%= product.brand.name %></td>
    <td><%= product.name %></td>
    <td><%= link_to 'Show', product %></td>
    <td><%= link_to 'Edit', edit_product_path(product) %></td>
    <td><%= link_to 'Destroy', product, method: :delete, data: { confirm: 'Are you sure?' } %></td>
  </tr>
<% end %>

I see that two queries get run:

SELECT `products`.* FROM `products`  
SELECT `brands`.* FROM `brands`  WHERE `brands`.`id` IN (1, 2)

I'm wondering why ActiveRecord doesn't just run the following query?

select products.*, brands.name
from products
join brands
on brands.id = products.brand_id

I think I must be missing something-- because I'm quite certain when I do something similar using .NET's Entity Framework and use Includes, there would only be one query run.

Am I missing something?

Answer 1

I believe the difference between ActiveRecord and Entity Framework is that Entity Framework specifically stores the schema somewhere where as ActiveRecord needs to query the database to get the schema.

So, and I could be wrong, if ActiveRecord did a query like

select products.*, brands.*
from products
join brands
on brands.id = products.brand_id

It would not know where the product fields ended and the brands fields began.