Added documentation for new Base.find API and eager association loading

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@1209 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activerecord/CHANGELOG

@@ -1,5 +1,27 @@
 *SVN*
 
+* Added eager loading of associations as a way to solve the N+1 problem more gracefully without piggy-back queries. Example:
+
+    for post in Post.find(:all, :limit => 100)
+      puts "Post:            " + post.title
+      puts "Written by:      " + post.author.name
+      puts "Last comment on: " + post.comments.first.created_on
+    end
+  
+  This used to generate 301 database queries if all 100 posts had both author and comments. It can now be written as:
+  
+    for post in Post.find(:all, :limit => 100, :include => [ :author, :comments ])
+ 
+  ...and the number of database queries needed is now 1.
+
+* Added new unified Base.find API and deprecated the use of find_first and find_all. See the documentation for Base.find. Examples:
+
+    Person.find(1, :conditions => "administrator = 1", :order => "created_on DESC")
+    Person.find(1, 5, 6, :conditions => "administrator = 1", :order => "created_on DESC")
+    Person.find(:first, :order => "created_on DESC", :offset => 5)
+    Person.find(:all, :conditions => [ "category IN (?)", categories], :limit => 50)
+    Person.find(:all, :offset => 10, :limit => 10)
+
 * Fixed PostgreSQL usage of fixtures with regards to public schemas and table names with dots #962 [gnuman1@gmail.com]
 
 * Fixed that fixtures were being deleted in the same order as inserts causing FK errors #890 [andrew.john.peters@gmail.com]

activerecord/lib/active_record/associations.rb

@@ -108,6 +108,41 @@ module ActiveRecord
     #   project.milestones(true).size  # fetches milestones from the database
     #   project.milestones             # uses the milestone cache
     #
+    # == Eager loading of associations
+    #
+    # Eager loading is a way to find objects of a certain class and a number of named associations along with it in a single SQL call. This is
+    # one of the easiest ways of to prevent the dreaded 1+N problem in which fetching 100 posts that each needs to display their author
+    # triggers 101 database queries. Through the use of eager loading, the 101 queries can be reduced to 1. Example:
+    #
+    #   class Post < ActiveRecord::Base
+    #     belongs_to :author
+    #     has_many   :comments
+    #   end
+    #
+    # Consider the following loop using the class above:
+    #
+    #   for post in Post.find(:all, :limit => 100)
+    #     puts "Post:            " + post.title
+    #     puts "Written by:      " + post.author.name
+    #     puts "Last comment on: " + post.comments.first.created_on
+    #   end 
+    #
+    # To iterate over these one hundred posts, we'll generate 201 database queries. Let's first just optimize it for retrieving the author:
+    #
+    #   for post in Post.find(:all, :limit => 100, :include => :author)
+    #
+    # This references the name of the belongs_to association that also used the :author symbol, so the find will now weave in a join something
+    # like this: LEFT OUTER JOIN authors ON authors.id = posts.author_id. Doing so will cut down the number of queries from 201 to 101.
+    #
+    # We can improve upon the situation further by referencing both associations in the finder with:
+    #
+    #   for post in Post.find(:all, :limit => 100, :include => [ :author, :comments ])
+    #
+    # That'll add another join along the lines of: LEFT OUTER JOIN comments ON comments.post_id = posts.id. And we'll be down to 1 query.
+    # But that shouldn't fool you to think that you can pull out huge amounts of data with no performance penalty just because you've reduced
+    # the number of queries. The database still needs to send all the data to Active Record and it still needs to be processed. So its no
+    # catch-all for performance problems, but its a great way to cut down on the number of queries in a situation as the one described above.
+    #
     # == Modules
     #
     # By default, associations will look for objects within the current module scope. Consider:

activerecord/lib/active_record/base.rb

@@ -299,18 +299,31 @@ module ActiveRecord #:nodoc:
       #
       # All approaches accepts an option hash as their last parameter. The options are:
       #
-      # * <tt>:conditions</tt>: 
-      # * <tt>:order</tt>: 
-      # * <tt>:limit</tt>: 
-      # * <tt>:offset</tt>: 
-      # * <tt>:joins</tt>: 
-      # * <tt>:include</tt>: 
+      # * <tt>:conditions</tt>: An SQL fragment like "administrator = 1" or [ "user_name = ?", username ]. See conditions in the intro.
+      # * <tt>:order</tt>: An SQL fragment like "created_at DESC, name".
+      # * <tt>:limit</tt>: An integer determining the limit on the number of rows that should be returned.
+      # * <tt>:offset</tt>: An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
+      # * <tt>:joins</tt>: An SQL fragment for additional joins like "LEFT JOIN comments ON comments.post_id = id". (Rarely needed).
+      # * <tt>:include</tt>: Names associations that should be loaded alongside using LEFT OUTER JOINs. The symbols named refer
+      #   to already defined associations. See eager loading under Associations.
       #
-      # Examples:
+      # Examples for find by id:
       #   Person.find(1)       # returns the object for ID = 1
       #   Person.find(1, 2, 6) # returns an array for objects with IDs in (1, 2, 6)
       #   Person.find([7, 17]) # returns an array for objects with IDs in (7, 17)
       #   Person.find([1])     # returns an array for objects the object with ID = 1
+      #   Person.find(1, :conditions => "administrator = 1", :order => "created_on DESC")
+      #
+      # Examples for find first:
+      #   Person.find(:first)  # returns the first object fetched by SELECT * FROM people
+      #   Person.find(:first, :conditions => [ "user_name = ?", user_name])
+      #   Person.find(:first, :order => "created_on DESC", :offset => 5)
+      #
+      # Examples for find all:
+      #   Person.find(:all)    # returns an array of objects for all the rows fetched by SELECT * FROM people
+      #   Person.find(:all, :conditions => [ "category IN (?)", categories], :limit => 50)
+      #   Person.find(:all, :offset => 10, :limit => 10)
+      #   Person.find(:all, :include => [ :account, :friends ])
       def find(*args)
         options = extract_options_from_args!(args)