Grid:

class TimeEntriesGrid < BaseGrid

  #
  # Scope
  #

  scope do
    User.select(
      "users.name, projects.name as project_name, accounts.name as account_name, sum(time_entries.hours) as report_hours"
    ).joins(:time_entries => {:project => :account}).group("projects.name", "users.name", "accounts.name").order("users.name")

  end


  #
  # Filters
  #

  filter(:project_id, :enum,
    :select => lambda {Project.all.map {|p| [p.name, p.id]}},
    :multiple => true,
    :include_blank => false
  ) do |value|
    self.where(:time_entries => {:project_id => value})
  end


  filter(:year, :enum,
         :select => lambda { TimeEntry.all.any? ? (TimeEntry.minimum(:date).year..TimeEntry.maximum(:date).year) : []},
    :include_blank => false,
    :default => lambda {Date.today.year}
  ) do |value|
    self.where(["extract(year from time_entries.date) = ?", value.to_i])
  end

  filter(:month, :enum,
         :select => Date::MONTHNAMES[1..12].enum_for(:each_with_index).collect {|name, index| [name, index + 1]},
         :include_blank => false,
         :default => lambda {Date.today.month}
        ) do |value|

    self.where(["extract(month from time_entries.date) = ?", value.to_i])
  end


  #
  # Columns
  #

  column(:user_name, :header => "Developer", :order => "users.name") do
    self.name
  end
  column(:project_name, :header => "Project", :order => "projects.name")
  column(
    :account_name,
    :header => "Company",
    :order => "accounts.name",
  ) do |model|
    format(model.account_name) do
      render :partial => "time_entries/company", :locals => {:model => model}
    end
  end

  column(:report_hours, order: "sum(time_entries.hours)")

end

Controller:

class TimeEntriesController < ApplicationController

  def index
    @time_entries_grid = TimeEntriesGrid.new(params[:g]) do |scope|
      scope.page(params[:page] )
    end
  end
end

View:

<h3>Time Entries Grid</h3>


<div class="left">
  <%= datagrid_form_for @time_entries_grid, url: time_entries_path %>
  <br/>

  <%= datagrid_table(@time_entries_grid) %>
  <%= paginate @time_entries_grid.assets %>

</div>

<div class="right">
  <%= render :partial => "shared/source", :object => @time_entries_grid %>
</div>