From: zdennis <zdennis@...>
Date: 2006-08-29T11:28:27+09:00
Subject: Re: ActiveRecord, has_many and _count ...

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�ric DUMINIL wrote:

> I just would like to know: The right way to create a new company by updating 
> Country.find(id).companies.size is by doing 
> Country.find(id).companies.create . Right?

This is one way of doing it. Although if you have the id of the Country
you can just say:
  company = Company.create( :company_name=>'ABC Co', :country_id=>1 )
OR
  company = Company.new( :company_name=>'ABC Co', :country_id=>1 )
  company.save

Both of these will update the country countries_counter because you have
defined this in your belongs_to relationship.


> 
> 1) But what if I'd like to keep in cache companies.size for the category in 
> which the new company is put? I can't create a company simultaneously using 
> two parents!

This can be solved using create or save on Company itself, rather then
doing Country.find.companies.create. Since you have defined the
belongs_to relationship and specified the counter cache ActiveRecord
will be smart enough to update both of them. The following example will
work:
  Company.create :company_name=>'ABC', :country_id=>1, :category_id=>1
> 
> 2) When I edit the properties of a company, let's say moving from Japan to 
> Italy, how could I update  Japan.companies_count and Italy.companies_count?

If you edit the properties of a company and save it ActiveRecord will
correctly increment/decrement the counter cache fields that you have
placed on your tables.

Using :refresh will ensure that you get the most recent data from the
database.

> 
> 3) Is there a Rails' way to cache the number of Employees directly in 
> Company.employees_count and Category.employees_count (i.e. through 
> a "has_many_many" relation)? That would prevent iterating through every 
> company of a given country/category to get its .employees_count!

I don't know about the count via a another relationship, but you can
always count them at once rather then iterating over each company.

Category.count(
  :joins=>"INNER JOIN #{Company.table_name} ON " +
      "#{Company.table_name}.category_id=" +
      "#{Category.table_name}.id " +
      "INNER JOIN #{Employee.table_name} ON " +
      "#{Employee.table_name}.company_id=#{Company.table_name}.id",
  :group=>"#{Category.table_name}.id" )

This will give you an array of arrays, where each internal array is
employee_count, category_id:
  [ [ 4, 1 ], [ 2, 2 ], [ 30, 3 ] ]

It doesn't handle categories that do not have any employees although
that could be fixed.

This is perhaps not as simple of a solution you're looking for. Perhaps
someone else knows if you can do counter_cache using :through. ;)

Zach






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