Flashback time. We are in 1999 and you are coding in ASP thinking that this language is the future. You use ADODB recordsets to iterate over collections. You have a recordset containing some records from a “quotes” table. The “quotes” table contains a author column (varchar) and a body column (varchar). Now you want to display the results grouped by author name so it looks like this :

Georges Brassens
- Les filles quand ça dit “je t’aime”, c’est comme un second baptême
- Aucune idée sur terre n’est digne d’un trépas

Billie Holiday
- Don’t threaten me with love, baby. Let’s just go walking in the rain.
- I never hurt nobody but myself and that’s nobody’s business but my own.

Assuming your recordset is ordered by author, you do something like this (remember, we’re in 1999) :

<TABLE style="color:fushia;font-style:MSONormal generated=frontpage">
<%while not objRS.eof %>
<%if(current_author <> objRS("author"))%>
<h1><%=objRS("author")%></h1>
<%end if%>
<Tr><td><%=objRS("body")</td></TR>
<%current_author = objRS("author")%>
<%objRS.MoveNext%>
<%loop%>
<%objRS.close%>
</TABLE>

Ok welcome back in 2008. ASP is dead. You are coding in rails and you want to do the same thing. How will you do it? Storing the author name in a buffer variable like in 1999? Not too sure about it.

This is a job for Enumerable#group_by (Enumerable is a module that is mixed in the Array class)

all_quotes = Quote.find(:all)
@authors = all_quotes.group_by(&:author)

Enumerable#group_by will create different sets of quotes based on the “author” values. Why the “&” sign? It’s because group_by expects a block. I could have done it this way : @authors = all_quotes.group_by{|quote| quote.author}

So @authors is now a hash that will look like this:

{“George Brassens” => [#<Quote id:131 ...>, #<Quote id:331 ...>], “Billie Holiday” => [#<Quote id:111 ...>, #<Quote id:911 ...>] }

Now you can iterate over it like this :

@authors.each_pair do |author_name, quotes|
  <h1><%=author_name%></h1>
  <%quotes.each do |quote| %>
    - <%=quote.body%><br />
  <%end%>
<%end%>

One more thing to note : @authors is a hash, and hashes cannot be ordered. If you want to display quotes by sorted author name, you could do this :

@authors.keys.sort.each do |author_name|
  <h1><%=author_name%></h1>
  <%@authors[author_name].each do |quote|%>
    - <%=quote.body%><br />
  <%end%>
<%end%>

Note : Enumerable#group_by does not exist in ruby 1.8, it only exists in Rails. The method will be in ruby 1.9 however.