This is how I implement budgets for ledger web

20121108220000_add_budget_tables.rb

Sequel.migration do
  up do
    create_table(:budget_periods) do
      String :account
      BigDecimal :amount
      Date :from_date
      Date :to_date
    end

    create_or_replace_view :budget_months, "select xtn_month, account, amount from (select distinct xtn_month from accounts_months) x cross join budget_periods where xtn_month between budget_periods.from_date and (coalesce(budget_periods.to_date, now()::date))"
  end
  down do
    drop_view :budget_months
    drop_table :budget_periods
  end
end

budget.erb

<div class="page-header">
  <h1>Budget Summary</h1>
</div>
<div class="row">
  <div class="span16">
    <%= table BudgetSummaryReport.run, :links => {/Month/ => '/reports/budget_detail?month=:0'} %>
  </div>
</div>

budget.rb

class BudgetSummaryReport < LedgerWeb::Report
  def self.run(limit=nil)
    limit_sql = limit ? "limit #{limit.to_i}" : ""
    month_where = params[:month] ? 'xtn_month = :month' : '1 = 1'
    from_query("""
      with budget_summary as (
      select
          b.xtn_month,
          x.account as account,
          x.amount as expense,
          b.amount as budgeted
      from
      budget_months b
      left outer join (
          select
              xtn_month,
              account,
              sum(amount) as amount
          from
              expenses
          where
              account in (select distinct account from budget_periods)
          group by
              xtn_month,
              account
      ) x on x.account = b.account and x.xtn_month = b.xtn_month
      order by
         b.xtn_month,
         x.account
      )
      select
          xtn_month as \"Month\",
          sum(budgeted) as \"Budget\",
          sum(expense) as \"Spent\",
          sum(budgeted) - sum(expense) as \"Diff\"
      from
          budget_summary
      where
          #{month_where}
      group by
          xtn_month
      order by
          xtn_month desc
      #{limit_sql}
    """)
  end
end

budget_detail.erb

<% default 'month', Date.today.strftime('%Y-%m-01') %>
<% @query = query do %>
select
    b.account as "Account",
    b.amount as "Budget",
    coalesce(x.amount, 0) as "Spent",
    b.amount - coalesce(x.amount, 0) as "Diff"
from
(select * from budget_months where xtn_month = :month) b
left outer join (
    select
        xtn_month,
        account,
        sum(amount) as amount
    from
        expenses
    where
        account in (select distinct account from budget_periods)
        and xtn_month = :month
    group by
        xtn_month,
        account
) x using (account)
order by
   b.xtn_month,
   x.account
<% end %>
<% @budget_summary = BudgetSummaryReport.run(1) %>
<%
  this_month = Date.strptime(LedgerWeb::Report.params[:month], "%Y-%m-%d")
  @prev_month = (this_month << 1).strftime("%Y-%m-01")
  @next_month = (this_month >> 1).strftime("%Y-%m-01")
%>
<div class="page-header">
  <h1>Budget Detail <small><%= LedgerWeb::Report.params[:month] %> <a href="/reports/budget_detail?month=<%= @prev_month %>">prev</a> <a href="/reports/budget_detail?month=<%= @next_month %>">next</a></small></h1>
</div>
<div class="row">
  <div class="span16">
    <%= table(@budget_summary) %>
  </div>
</div>
<div class="row">
  <div class="span16">
    <%= table @query, :links => {/Account/ => "/reports/register?account=:0&month=#{LedgerWeb::Report.params[:month]}" } %>
  </div>
</div>

budget_periods.csv

config.rb

LedgerWeb::Config.new do |config|
  config.add_hook :after_load do |db|
    puts "Loading budget"

    db["delete from budget_periods"].delete

    path = File.join(File.dirname(ENV['LEDGER_FILE']), "budget_periods.csv")
    return unless File.exists?(path)

    CSV.foreach(path, :headers => true) do |row|
      db[:budget_periods].insert(row.to_hash)
    end

    puts "Done loading budget"
  end
end