controller-code.rb

# creating a fake list
cities = ['Miami', 'Diadema', 'NYC', 'Sao Paulo']
fruits = ['Manana', 'Molango', 'Bacate', 'Acabaxi']

list = []
cities.each{|city| fruits.each{|fruit| list.push({city: city, product: fruit, count: rand(10..100)}) }}

puts list

# this will hold the crosstab
@myCities = {}

# this will hold the unique products for the header
@uniqProducts = {}

# Creating crosstab structure
list.each{|row|
  cityName    = row[:city]
  productName = row[:product]
  @uniqProducts[ productName ] = true;

  # cria um hash vazio se aquela chave ainda nao existe  
  @myCities[ cityName ] = {} if @myCities[ cityName ].nil?

  @myCities[ cityName ][ productName ] = row[:count]
}

template.html.erb

<table>
  <thead>
    <tr>
      <th>Cities</th>
    <% @uniqProducts.keys.each do |productName| %>
      <th><%= productName %></th>
    <% end %>
    </tr>
  </thead>
  <tbody>
    <% @myCities.each_pair do |cityName, cityProducts| %>
    <tr>
      <th><%= cityName %></th>
      <% @uniqProducts.keys.each do |productName| %>
      <td><%= cityProducts[ productName ] %></td>
      <% end #uniqProducts.keys %>
    </tr>
    <% end #myCities.each_with_keys %>
  </tbody>
</table>