rails group by

1，Enumerable#group_by():

<%
  employees = Employee.find(:all).group_by{|employee|
    employee.title
  }
%>
<% employees.each do |title, people| %>
  <h2><%= title %></h2>
  <ul>
    <% people.each do |person| %>
      <li><%= person.name %></li>
    <% end %>
  </ul>
<% end %>

2，Array#in_groups_of()

<table class="calendar">
  <% (1..DAYS_IN_MARCH).to_a.in_groups_of(7) do |group| %>
    <tr>
      <% group.each do |day| %>
        <td><%= day %></td>
      <% end %>
    </tr>
  <% end %>
</table>

Array#in_groups_of()可以有第二个参数，当一个group的某几个为空时值为该参数，否则为nil