title: add column to data.frame by another data.frame
body: <p>I have two dataframes in my program, one of which stores the information about purchases (including product IDs), the other stores information about products, (product IDs and brand names(as factor))
  I want to add to 1st dataframe column with brand names for each product.
  I want something like:</p>
  
  <pre><code>purchases$brand &lt;- products[purchases$product_id == products$id,]$brand 
  </code></pre>