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@@ -1,5 +1,7 @@
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 *2.0.0 [Preview Release]* (September 29th, 2007) [Includes duplicates of changes from 1.14.2 - 1.15.3]
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+* Make size for has_many :through use counter cache if it exists.  Closes #9734 [xaviershay]
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+
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 * Remove DB2 adapter since IBM chooses to maintain their own adapter instead.  [Jeremy Kemper]
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 * Extract Oracle, SQLServer, and Sybase adapters into gems.  [Jeremy Kemper]

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@@ -100,7 +100,9 @@
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       # calling collection.size if it has. If it's more likely than not that the collection does have a size larger than zero
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       # and you need to fetch that collection afterwards, it'll take one less SELECT query if you use length.
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       def size
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-        loaded? ? @target.size : count
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+        return @owner.send(:read_attribute, cached_counter_attribute_name) if has_cached_counter?
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+        return @target.size if loaded?
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+        return count
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       end
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       # Calculate sum using SQL, not Enumerable
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@@ -258,6 +260,14 @@
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         end
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         alias_method :sql_conditions, :conditions
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+
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+        def has_cached_counter?
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+          @owner.attribute_present?(cached_counter_attribute_name)
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+        end
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+
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+        def cached_counter_attribute_name
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+          "#{@reflection.name}_count"
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+        end
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     end
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   end
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 end

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@@ -457,6 +457,15 @@
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     assert !author.comments.loaded?
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   end
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+  uses_mocha('has_many_through_collection_size_uses_counter_cache_if_it_exists') do
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+    def test_has_many_through_collection_size_uses_counter_cache_if_it_exists
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+      author = authors(:david)
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+      author.stubs(:read_attribute).with('comments_count').returns(100)
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+      assert_equal 100, author.comments.size
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+      assert !author.comments.loaded?
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+    end
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+  end
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+
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   def test_adding_junk_to_has_many_through_should_raise_type_mismatch
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     assert_raise(ActiveRecord::AssociationTypeMismatch) { posts(:thinking).tags << "Uhh what now?" }
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   end