Question

In Rails while using activeRecord why are join queries considered bad.

For example

Here i’m trying to find the number of companies that belong to a certain category.

  class Company ActiveRecord::Base
    has_one :company_profile
  end  

Finding the number of Company for a particular category_id

number_of_companies = Company.find(:all, :joins=>:company_profile, :conditions=>["(company_profiles.category_id = #{c_id}) AND is_published = true"])   

How could this be better or is it just poor design?

  company_profiles = CompanyProfile.find_all_by_category_id(c_id)
  companies = []
  company_profiles.each{|c_profile| companies.push(c_profile.company) } 
 

Isn't it better that the first request creates a single query while i'd be running several queries for the second case.

Could someone explain why joins are considered to be bad practice in Rails

Thanks in advance

Answer 1

To my knowledge, there is no such rule. The rule is to hit the database as least as possible, and rails gives you the right tools for that, using the joins.

The example Sam gives above is exemplary. Simple code, but behind the scenes rails has to do two queries, instead of only one using a join.

If there is one rule that comes to mind, that i think is related, is to avoid SQL where possible and use the rails way as much as possible. This keeps your code database agnostic (as rails handles the differences for you). But sometimes even that is unavoidable.

It comes down to good database design, creating the correct indexes (which you need to define manually in migrations), and sometimes big nested structures/joins are needed.