title: Rails find Parent that has two Children with certain attribute
body: <p>I have over 100,000 objects in my database for different Product. Each Product has 4-6 Variants. Because of this, it is not easy to lazily edit large amount of data by iterating through everything. Because of this, I am trying to get only the exact number of Products I need.<p>
  So far, I can get all the Products that have a Variant with the size attribute 'SM'.
  <p>
  The hang up, is getting all the Products that have both a Variant with size 'MD' and 'SM'.
  <p>
  This is the code I am using <code>Product.joins(:variants).where('variants.size = ?', 'SM')</code>
  <p>
  I have tried adding <code>.where('variants.size = ?', 'MD')</code> to it, but that does work.</p>