[Rails] Re: Trying to get a list of households with one person in them

On Aug 23, 6:48 pm, Rob Biedenharn <R...@AgileConsultingLLC.com>
wrote:
> On Aug 23, 2010, at 6:17 PM, Sandy wrote:
>
>
>
>
>
> > On Aug 23, 5:05 pm, Rob Biedenharn <R...@AgileConsultingLLC.com>
> > wrote:
> >> On Aug 23, 2010, at 4:15 PM, Colin Law wrote:
> >>> On 23 August 2010 21:03, Bob Smith <bsm...@gmail.com> wrote:
> >>>> in my models, household has_many :people and people
> >>>> has_one :household
> >>>> household has a name field, and people has household_id
>
> >>> That should be person belongs_to household.
>
> >>>> the closest I got was:
> >>>> @test = Household.count :all, :group =>  
> >>>> 'people.household_id', :joins
> >>>> => :people
>
> >>>> this does group the families together and count the members,
> >>>> but this has no way of including only families of one
> >>>> I am assuming that the output from this could be used to get the  
> >>>> name
> >>>> field from households
>
> >>> It might be worth looking at counter_cache.
>
> >>> Colin
>
> >> That might not be a bad idea (or even running a
> >> Household.connection.select_value on the SQL).
>
> >> Well, the answer is going to be different for ActiveRecord 2.x and  
> >> 3.0
>
> >> Here's how it would look in SQL
>
> >> SELECT COUNT(households.id) FROM households
> >> INNER JOIN people ON people.household_id = households.id
> >> GROUP BY households.id
> >> HAVING COUNT(people.id) = 1
>
> >> In AR 2.x, that's probably:
>
> >> Household
> >> .count
> >> (:include
> >> =
>
> >> :people
> >> , :group=>'households.id', :having=>'COUNT(people.id)=1').first.first
>
> >> or since the join is simple and not truly needed:
> >> Person.count(:select => 'people.id', :group =>
> >> 'people.household_id', :having => 'COUNT(id)=1').first.first
>
> >> (the return will be an array of pairs [count,1] so [[count,
> >> 1]].first.first will be count)
>
> >> And in AR 3.0, something like:
>
> >> Household
> >> .select
> >> ('COUNT
> >> (households
> >> .id
> >> )').includes
> >> (:people
> >> ).group('households.id').having('COUNT(people.id)=1').to_a.size
> >> -or-
> >> Person
> >> .select
> >> ('COUNT(id)').group('household_id').having('COUNT(id)=1').to_a.size
>
> >> You might be thinking "Hmm, ActiveRecord doesn't seem to be well
> >> suited for a query like this"
>
> >> And I think you'd be right!
>
> >> -Rob
>
> >> Rob Biedenharn
> >> R...@AgileConsultingLLC.com        http://AgileConsultingLLC.com/
> >> r...@GaslightSoftware.com          http://GaslightSoftware.com/- 
> >> Hide quoted text -
>
> >> - Show quoted text -
>
> > As Colin (sort of) suggested, your Household model should contain
>
> > has_many :people
>
> > and your Person model should contain
>
> > belongs_to :household
>
> > Don't forget the colons, as shown above.
>
> > Next, since the foregoing relationship will allow you to determine the
> > number of People in each Household, I would stay away from trying to
> > create an SQL statement, and, instead, allow the controller to provide
> > the appropriate array to your view, as follows:
>
> > def single_person_household
> >  households = Household.all
> >  @households_with_one = Array.new
> >  for h in households do
> >    if h.people.count == 1.to_i
> >      @households_with_one << h
> >    end
> >  end
> > end
>
> But since Bob hasn't indicated the size of the Household and Person  
> data sets, I'd caution that performance of that code on a large number  
> of households could be dismal as it has to first read every household  
> into an object (i.e., memory) and then make a separate query to the  
> database to count the persons.
>
> Also, there's never a need to do 1.to_i (1 is already an integer).
>
> If you really wanted to add this as Ruby code (rather than trying to  
> leverage ActiveRecord directly, I'd suggest something at least a bit  
> more idiomatic if no more efficient.
>
> class Person < ActiveRecord::Base
>    belongs_to :household
> end
>
> class Household < ActiveRecord::Base
>    has_many :people
>
>    # reads all records, then a COUNT(*) query to determine people
>    def self.single_person_households
>      find(:all).select {|household| household.people.count == 1 }
>    end
>
>    # Or since the original question was for a count,
>    # not the actual records...
>    # Get just the COUNT of single person households
>    def self.count_single_person_households
>      connection.select_value(<<-ENDSQL)
>        SELECT COUNT(households.id) AS the_count
>        FROM households
>        INNER JOIN people ON people.household_id = household.id
>        GROUP BY households.id
>        HAVING COUNT(people.id) = 1
>      ENDSQL
>    end
> end
>
> puts Household.count_single_person_households
> Household.single_person_households.each do |household|
>    puts household.name
> end
>
> -Rob
>
>
>
> > NOTE:  If you create a view called single_person_household.html.erb,
> > then you will need a route that maps to that view in your routes.rb
> > file, such as
>
> > map.connect 'single_person_household', :controller =>
> > 'households', :action => 'single_person_household'
>
> > In your view, you will use @households_with_one.
>
> > By the way, before I post code, I test it, so I know that I am
> > actually answering the question which I understood to have been asked.
>
> > Sandy
>
> Rob Biedenharn          
> R...@AgileConsultingLLC.com        http://AgileConsultingLLC.com/
> r...@GaslightSoftware.com  http://GaslightSoftware.com/- Hide quoted text -
>
> - Show quoted text -

Rob,

I agree that SQL is more efficient than the Ruby code. However, not
every application requires the ultimate in database 'efficiency', and
my experience is that such efficiency, at the expense of coding time
and clarity, is often overrated

As far as the "original question" goes, he said, "I am assuming that
the output from this could be used to get the name field from
households." The fact that he wants the name field from the
households implies that he needs the array such that he can then
display the name field. The count, alone, doesn't supply the name
field.

Sandy

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