Document identity map inconsistency with associations, closes #474.

activerecord/lib/active_record/identity_map.rb

@@ -12,7 +12,34 @@ module ActiveRecord
   # In order to enable IdentityMap, set <tt>config.active_record.identity_map = true</tt>
   # in your <tt>config/application.rb</tt> file.
   #
-  # IdentityMap is disabled by default.
+  # IdentityMap is disabled by default and still in development (i.e. use it with care).
+  #
+  # == Associations
+  #
+  # Active Record Identity Map does not track associations yet. For example:
+  #
+  #   comment = @post.comments.first
+  #   comment.post = nil
+  #   @post.comments.include?(comment) #=> true
+  #
+  # Ideally, the example above would return false, removing the comment object from the
+  # post association when the association is nullified. This may cause side effects, as
+  # in the situation below, if Identity Map is enabled:
+  #
+  #   Post.has_many :comments, :dependent => :destroy
+  #
+  #   comment = @post.comments.first
+  #   comment.post = nil
+  #   comment.save
+  #   Post.destroy(@post.id)
+  #
+  # Without using Identity Map, the code above will destroy the @post object leaving
+  # the comment object intact. However, once we enable Identity Map, the post loaded
+  # by Post.destroy is exactly the same object as the object @post. As the object @post
+  # still has the comment object in @post.comments, once Identity Map is enabled, the
+  # comment object will be acciddently removed.
+  #
+  # This incosistency is meant to be fixed in future Rails releases.
   #
   module IdentityMap