execute joins manually without :include parameter


I have the next model with the follow associations:

class XmlGen < ActiveRecord::Base
  set_primary_key "pk"
  belongs_to :serie,
             :foreign_key => "series_fk"

class Serie < ActiveRecord::Base
  set_primary_key "pk"
  belongs_to :study, #Relaciones entre las clases Study y Series. A
study has many series.
             :foreign_key => "study_fk"
  has_many :xml_genes

I did the follow query:

@instances = Instance.find(:all, :include => [:serie,

but the query is so slow. I want to try do the query manually, with
the option :joins. How do I do this query, exactly, the
parameter :joins?

Thanks in advance!