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@@ -193,6 +193,7 @@ module ActiveRecord
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       end
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       def preload_has_one_association(records, reflection, preload_options={})
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+        return if records.first.send("loaded_#{reflection.name}?")
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         id_to_record_map, ids = construct_id_map(records)        
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         options = reflection.options
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         records.each {|record| record.send("set_#{reflection.name}_target", nil)}
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@@ -214,6 +215,7 @@ module ActiveRecord
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       end
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       def preload_has_many_association(records, reflection, preload_options={})
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+        return if records.first.send(reflection.name).loaded?
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         options = reflection.options
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         primary_key_name = reflection.through_reflection_primary_key_name
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@@ -271,6 +273,7 @@ module ActiveRecord
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       end
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       def preload_belongs_to_association(records, reflection, preload_options={})
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+        return if records.first.send("loaded_#{reflection.name}?")
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         options = reflection.options
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         primary_key_name = reflection.primary_key_name
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@@ -1248,6 +1248,11 @@ module ActiveRecord
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             association.target.nil? ? nil : association
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           end
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+          define_method("loaded_#{reflection.name}?") do
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+            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
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+            association && association.loaded?
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+          end
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+
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           define_method("#{reflection.name}=") do |new_value|
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             association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
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@@ -18,7 +18,7 @@ require 'models/developer'
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 require 'models/project'
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 class EagerAssociationTest < ActiveRecord::TestCase
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-  fixtures :posts, :comments, :authors, :categories, :categories_posts,
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+  fixtures :posts, :comments, :authors, :author_addresses, :categories, :categories_posts,
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             :companies, :accounts, :tags, :taggings, :people, :readers,
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             :owners, :pets, :author_favorites, :jobs, :references, :subscribers, :subscriptions, :books,
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             :developers, :projects, :developers_projects
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@@ -111,6 +111,46 @@ class EagerAssociationTest < ActiveRecord::TestCase
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     end
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   end
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+  def test_finding_with_includes_on_has_many_association_with_same_include_includes_only_once
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+    author_id = authors(:david).id
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+    author = assert_queries(3) { Author.find(author_id, :include => {:posts_with_comments => :comments}) } # find the author, then find the posts, then find the comments
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+    author.posts_with_comments.each do |post_with_comments|
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+      assert_equal post_with_comments.comments.length, post_with_comments.comments.count
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+      assert_equal nil, post_with_comments.comments.uniq!
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+    end
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+  end
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+
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+  def test_finding_with_includes_on_has_one_assocation_with_same_include_includes_only_once
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+    author = authors(:david)
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+    post = author.post_about_thinking_with_last_comment
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+    last_comment = post.last_comment
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+    author = assert_queries(3) { Author.find(author.id, :include => {:post_about_thinking_with_last_comment => :last_comment})} # find the author, then find the posts, then find the comments
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+    assert_no_queries do
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+      assert_equal post, author.post_about_thinking_with_last_comment
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+      assert_equal last_comment, author.post_about_thinking_with_last_comment.last_comment
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+    end
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+  end
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+
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+  def test_finding_with_includes_on_belongs_to_association_with_same_include_includes_only_once
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+    post = posts(:welcome)
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+    author = post.author
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+    author_address = author.author_address
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+    post = assert_queries(3) { Post.find(post.id, :include => {:author_with_address => :author_address}) } # find the post, then find the author, then find the address
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+    assert_no_queries do
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+      assert_equal author, post.author_with_address
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+      assert_equal author_address, post.author_with_address.author_address
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+    end
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+  end
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+
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+  def test_finding_with_includes_on_null_belongs_to_association_with_same_include_includes_only_once
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+    post = posts(:welcome)
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+    post.update_attributes!(:author => nil)
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+    post = assert_queries(2) { Post.find(post.id, :include => {:author_with_address => :author_address}) } # find the post, then find the author which is null so no query for the address
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+    assert_no_queries do
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+      assert_equal nil, post.author_with_address
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+    end
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+  end
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+
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   def test_loading_from_an_association
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     posts = authors(:david).posts.find(:all, :include => :comments, :order => "posts.id")
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     assert_equal 2, posts.first.comments.size

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@@ -17,6 +17,8 @@ class Author < ActiveRecord::Base
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       proxy_target
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     end
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   end
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+  has_one  :post_about_thinking, :class_name => 'Post', :conditions => "posts.title like '%thinking%'"
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+  has_one  :post_about_thinking_with_last_comment, :class_name => 'Post', :conditions => "posts.title like '%thinking%'", :include => :last_comment
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   has_many :comments, :through => :posts
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   has_many :comments_containing_the_letter_e, :through => :posts, :source => :comments
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   has_many :comments_with_order_and_conditions, :through => :posts, :source => :comments, :order => 'comments.body', :conditions => "comments.body like 'Thank%'"

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@@ -13,6 +13,7 @@ class Post < ActiveRecord::Base
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   end
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   belongs_to :author_with_posts, :class_name => "Author", :foreign_key => :author_id, :include => :posts
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+  belongs_to :author_with_address, :class_name => "Author", :foreign_key => :author_id, :include => :author_address
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   has_one :last_comment, :class_name => 'Comment', :order => 'id desc'
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