ruby on rails group by with null values problem

I have an hour table in witch I store user time tracking information, the table consists from the following cells

project_id
task_id (optional can be null)
worker_id
reported_date
working_hours

each worker enters several records per day so generally the table is looking like this

id project_id  worker_id task_id   reported_date  working hours;    
== =========== ========= ========= =============  ==============
1  1           1         1         10/10/2011      4                 
2  1           1         1         10/10/2011     14                 
3  1           1                   10/10/2011      4                 
4  1           1                   10/10/2011     14                 

the task_id is not a must field so there can be times when the user is not selecting it and their task_id cell is empty

now i need to display the data by using group by clause

so the result will be something like this:

project_id worker_id task_id   working hours
========== ========= ========= ==============
1          1         1         18                 
1          1                   18            

I did the following group by condition:

@group_hours = Hour.group('project_id,worker_id,task_id)').
      select('project_id, task_id ,worker_id,sum(working_hours) as开发者_C百科 
         working_hours_sum')

My view looks like this

<% @group_hours.each do |b| %>
     <tr>
       <td><%= b.project.name if b.project %></td>
        <td><%= b.worker.First_name if b.worker %></td>
        <td><%= b.task.name if b.task %></td>
        <td class="center"><%= b.working_hours_sum %></td>
       <td></td>
     </tr>
<% end %>

This it is working but only if the task_id is not null when task id is null it present all the records without grouping them like this

project_id  worker_id task_id   working hours
=========== ========= ========= ==============
1           1         1         18                 
1           1                    4            
1           1                   14    

I will appreciate any kind of solution to this problem

Assuming you are using mysql you can use the COALESCE function to work with nulls, by changing the null value to something else

e.g.

select COALESCE(colname,0) from table where COALESCE(colname,0) > 1;

IFNULL() is another option