title: Devise `find_first_by_auth_conditions` explanation
body: <p>I am having a tough time understanding this code from <a href="https://github.com/plataformatec/devise" rel="nofollow">Devise</a>, even though I've read the documentation and done some research.</p>
  
  <pre><code>def self.find_first_by_auth_conditions(warden_conditions)
    conditions = warden_conditions.dup
    signin = conditions.delete(:signin)
    where(conditions).where(["lower(username) = :value OR lower(email) =
      :value", {:value =&gt; signin.downcase }]).first
  end
  </code></pre>
  
  <p>Please explain the components of this portion of the above method:</p>
  
  <pre><code>where(conditions).where(["lower(username) = :value OR lower(email) =
    :value", {:value =&gt; signin.downcase }]).first
  </code></pre>