Rails, has_many and joins

Question

Hello! I have three models, User, Type and TypeDescription, and as you can see below, each User may have many types, but each type has only one description. So as optimization, I thought each TypeDescription should be joined with Type via JOIN in sql, so I used default_scope and defined join, and that works when I get type via Type.find( id ), but when I use user = User.find( 1 ), each type in user.types doesn't have data from TypeDescription, because default_scope just adds defined options to methods like find, all etc. So what I'm looking for is solution to have this work in my situation, so what I want is, when I get certain or all users, I want to have all User's types and each type should have data from TypeDescription.

so, code is:

class User
  has_many :types
end

class Type
  has_one :type_description

  default_scope :joins => :type_description
end

class TypeDescription
  belongs_to :type
end

Thanks!

Answer 1

if you want to have the TypeDescription with the Type, you need to use :include, not :joins

Answer 2

Since rails 2.1, the "include" has changed to generate multiple query instead of just one query. The reason is that for large table, a join can produce lots of duplicated rows, which puts the overhead on the rails.

here is a good article to find more info. In your one-to-one situation, if you still want "include" to perform "join", the article did mention there is a filtering option but i can't find it myself.

either way, amikazmi is right, performance wise, it probably does not matter for now.