Rails - Aug 2007 - trying to find records that haven't been updated in 2 weeks

My app has some students that have attendances. I''m trying to find
students who have not attended class in the last 2 weeks. Here is my
code that is not working:

students = @school.students.active
@missed_attnd = []
students.each do |student|
  unless student.attendances.empty?
    @missed_attnd << student if student.attendances.first.date
<Date.today - 2.weeks
  end
end

An attendance has a date column. Any idea how I can get this to work?


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jko,

Databases can''t be assumed to retrieve rows in any specific order, so
student.attendances.first cannot be assumed to be the most recent
attendance.

In your loop, something like
  student.attendances.maximum(:date)
should give you the last date they attended. Then you can compae in
your loop.

However, note that this kind of loop is very expensive in database
terms (a lookup for each student).  I''m guessing you''ll be
asking this
question (ie. this query) more frequently than you''re updating the
database.  So it might make sense to optimise a little.

One approach is to add a column into attendances table called
"most_recent".  And in your model do this add filters to set
"most_recent" to true when adding a new record, and then setting the
previous most_recent to false.

Its more work on an insert, but a lot easier to manage a lookup.

missed_students = Student.attendances.find(:conditions =>
''students.active = true and attendances.most_recent = true and
attendances.date <= ?'', Date.today - 2.weeks)

So one database operation and you have a list of students missing
attendance.

Hope that helps a little,

Cheers, --Kip

On Aug 10, 5:50 am, jko170
<jko...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> My app has some students that have attendances. I''m trying to find
> students who have not attended class in the last 2 weeks. Here is my
> code that is not working:
>
> students = @school.students.active
> @missed_attnd = []
> students.each do |student|
>   unless student.attendances.empty?
>     @missed_attnd << student if student.attendances.first.date
<> Date.today - 2.weeks
>   end
> end
>
> An attendance has a date column. Any idea how I can get this to work?

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Thank you very much! I decided to go with the maximum(:date) because I
will only use this functionality on the dashboard. If the performance
is to slow then I''ll switch to your optimized suggestion.


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