Trying to get a list of households with one person in them

rubyonrails-talk
1

in my models, household has_many :people and people has_one :household household has a name field, and people has household_id

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

2

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

3

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

4

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

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

5

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 Rob@AgileConsultingLLC.com http://AgileConsultingLLC.com/ rab@GaslightSoftware.com http://GaslightSoftware.com/

6

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

7

Worked perfectly again Sandy. Thanks

Bob