Rails: Inner joins by association names (See related posts)

From http://habtm.com/articles/2006/05/10/inner-joins-by-association-names:

We can enjoy eager loading with association names.

Member.find(:all, :include=>[:group, :profile])

But :joins option forces us to write raw SQL like this.

Member.count(:joins=>"INNER JOIN groups ON groups.id = members.group_id ...")

Why should we do it by hand although those AR classes know their reflections? It’s too boring and painful, and far from DRY.
expand join query

So, why don’t you put this code to ‘vendor/plugins/expandjoinquery/init.rb’?

class ActiveRecord::Base
  class << self
    private
    def add_joins!(sql, options, scope = :auto)
      scope = scope(:find) if :auto == scope
      join = (scope && scope[:joins]) || options[:joins]
      sql << " #{expand_join_query(join)} " if join
    end

    def expand_join_query(*joins)
      joins.flatten.map{|join|
        case join
        when Symbol
          ref = reflections[join] or
            raise ActiveRecord::ActiveRecordError, "Could not find the source association :#{join} in model #{self}"
          case ref.macro
          when :belongs_to
            "INNER JOIN %s ON %s.%s = %s.%s" % [ref.table_name, ref.table_name, primary_key, table_name, ref.primary_key_name]
          else
            "INNER JOIN %s ON %s.%s = %s.%s" % [ref.table_name, ref.table_name, ref.primary_key_name, table_name, primary_key]
          end
        else
          join.to_s
        end
      }.join(" ")
    end
  end
end

Now, we can enjoy :joins option as same as :include one!

# simply inner join
Member.find(:first, :joins=>:group)
=> SELECT * FROM members INNER JOIN groups ON groups.id = members.group_id LIMIT 1

# two inner joins
Member.count(:joins=>[:group, :profile])
=> SELECT count(*) AS count_all FROM members
   INNER JOIN groups   ON groups.id = members.group_id
   INNER JOIN profiles ON profiles.member_id = members.id 

# symbol and raw SQL
Member.find(:all, :joins=>[:group, "INNER JOIN addresses USING(address_id)"])
=> SELECT * FROM members
   INNER JOIN groups ON groups.id = members.group_id 
   INNER JOIN addresses USING(address_id) 

restrictions

This is just an idea and not deeply considered yet. So, there are following restrictions.

* cascading is not supported
* not work with :include option (i think)

I hope someone will polish this up :)