<td title="id:<%= alert.id %>">Hello</td>

def admin?
  # check if this user is an admin
end

<%= content_tag :td, 'Hello', title: ("id:#{alert.id}" if admin?) %>

def admin?
  # check if this user is an admin
end

def generate_title(this)
    if admin?
      "id:#{this}"
    end
  end

<td title="<%= generate_title(alert.id) %>">Hello</td>

<td <%="title="id:#{alert.id}"" if admin?%>>Hello</td>

<% if admin? %>
  <td title="id:<%= alert.id %>">Hello</td>
<% end %>