When to use @ and when to use self

Rails
1

hey there,
i am seeking some general info about how to get at a certain value and
how to get that value in a method.

for example, lets say i have a machine.
each machine has_many :sensors
so sensor belongs_to :machine
sensor also has_many :reports
each report has a duration field that is then number of seconds since
the last report of a certain value
now if i wanted to add the seconds together for a total when in a
certain status, i would do something like this.
total = 0
for row in results
duration = row[0]
value = row[1]
if value == ‘on’:
total += duration

of course in rails we have sensor.value, sensor.duration
so, anyway. i want to list in a table these figures.

so i need a method in sensor.rb to do this, right

so if i have for machine in machines do
<%= machine.sensor.total %>

how do i declare that ?
in the Sensor class, do i have
def total
i dont know what to put here to get the right sensor

and is it def total, or is it def self.total ?

thanks

so for sensor in machine.sensors |sensor| do

2

You want to do

def total
…
end

because you want (I assume) to get the total for that particular
instance of Sensor.
You would then want to do something like

def total
reports.sum ‘duration’
end

Fred

3

On 11/21/06, Frederick C. [email protected] wrote:

You would then want to do something like

Hey thanks a lot. I appreciate it much.
i have not seen the duration.sum method before, i assume it adds all the
integers in that db field for the reports table ? thats cool too, i cant
use
it for this, because i can only add the ones where the status ==
‘running’
but still , good thing for my notes.

again, thanks

4

On 11/21/06, Frederick C. [email protected] wrote:

again, thanks

wow, i am so digging this. that line replaced 9 lines of php
this is really exciting, thanks
sk

5

shawn bright wrote:

i have not seen the duration.sum method before, i assume it adds all the
integers in that db field for the reports table ? thats cool too, i cant
use
it for this, because i can only add the ones where the status ==
‘running’
but still , good thing for my notes.

again, thanks

Sure you can: reports.sum :duration, :conditions => [‘status = ?’]

It’s basically the same as Report.sum, but scoped to reports that belong
to that object. You can also do joins, group by etc… in the usual way.

Fred