title: Elixir map check if not empty and key exists
body: <p>I am trying to figure out a way to check if the params hash in a Phoenix app (using Elixir) has a particular key or not.</p>
  
  <p>In the below changeset function in a model, the params is defaulted to :empty. </p>
  
  <pre><code>def changeset(model, params \\ :empty) do
  </code></pre>
  
  <p>I need to figure out if a key named :username exists in the hash.</p>