title: Rails ActiveRecord group_by & sum db results for use with Lazy HighCharts
body: <p>I am completely new to RoR/Ruby and i am using Lazy High Charts gem to generate some purdy charts based on some database information.</p>
  <p>I have tried the answers that were provided in a previous question but i am still a bit confused as to how to do this..</p>
  <p>I need to sum amount_used, and billed_amount and group by month/year (e.g; Aug/2012)</p>
  <p>The end result will be something similar to a dual axis chart with two series &quot;Amount Used&quot;, and &quot;Cost&quot;.. This information is specific to a certain account_id.</p>
  <p><img src="https://i.stack.imgur.com/VwUYZ.png" alt="enter image description here" /></p>
  <h2>Invoices table</h2>
  <pre><code>+---------------+--------------+------+-----+---------+----------------+
  | Field         | Type         | Null | Key | Default | Extra          |
  +---------------+--------------+------+-----+---------+----------------+
  | id            | int(11)      | NO   | PRI | NULL    | auto_increment |
  | account_id    | int(11)      | YES  |     | NULL    |                |
  | invoice_date  | varchar(255) | YES  |     | NULL    |                |
  | amount_used   | float        | YES  |     | NULL    |                |
  | billed_amount | float        | YES  |     | NULL    |                |
  | comments      | text         | YES  |     | NULL    |                |
  | created_at    | datetime     | NO   |     | NULL    |                |
  | updated_at    | datetime     | NO   |     | NULL    |                |
  +---------------+--------------+------+-----+---------+----------------+
  </code></pre>
  <h2>Controller Chart Code</h2>
  <pre><code>@account = Account.find(params[:id])
  @invoices = Invoice.where(&quot;account_id = #{@account.id}&quot;).order(&quot;invoice_date DESC&quot;)
  
  @h = LazyHighCharts::HighChart.new('area') do |f|
    f.options[:chart][:defaultSeriesType] = &quot;area&quot;
    #Sample dates right now, should be the grouped_by :invoice_date
    f.xAxis( :categories =&gt; ['May', 'Jun', 'Jul'] )
    f.yAxis([
      {
        :title =&gt; { :text =&gt; &quot;Amount Used&quot; }
      },
      {
        :title =&gt; { :text =&gt; &quot;Cost&quot; },
        :opposite =&gt; true
      }
    ])
    #Sample data right now, should be the summed amounts of the :amount_used correpsonding for each above grouped invoice_date
    f.series(:name =&gt; &quot;Amount Used&quot;, :data =&gt; [100,300,500] )
    #Sample data right now, should be the summed amounts of the :billed_amount correpsonding for each above grouped invoice date
    f.series(:name =&gt; &quot;Cost&quot;, :yAxis =&gt; 1, :data =&gt; [200,400,600] )
  end 
  </code></pre>