title: Join data frames based on item presence in a transaction
body: <p>I have two data frames that I am trying to merge. The first table is a list of items and associated data such as:</p>
  
  <pre><code>items &lt;- data.frame(
    item_code = c(1111, 2222, 3333, 4444),
    item_category = c("cata","catb","catc","catd")
    )
  </code></pre>
  
  <p>And the second is a list of transactions:</p>
  
  <pre><code>transactions &lt;- data.frame(
    tran_code = c('aaaa', 'bbbb', 'cccc', 'dddd'),
    tran_items = c("1111,1111,2222","3333,2222","1111,4444,4444","3333")
    )
  </code></pre>
  
  <p>I am trying to create a column containing in each cell a list of transactions that the item appears in, like so:</p>
  
  <pre><code>view(final_df)
  
  item_code item_category in_trans
  1111      "cata"        "aaaa,cccc"
  2222      "catb"        "aaaa,bbbb"
  3333      "catc"        "bbbb,dddd"
  4444      "catd"        "cccc"
  </code></pre>
  
  <p>Can anyone provide suggestions on how to achieve this?</p>