diff --git a/app/models/conversation_participant.rb b/app/models/conversation_participant.rb
index b0f2d94e95e..05f66b67fdc 100644
--- a/app/models/conversation_participant.rb
+++ b/app/models/conversation_participant.rb
@@ -266,14 +266,16 @@ class ConversationParticipant < ActiveRecord::Base
   end
 
   def move_to_user(new_user)
-    conversation.conversation_messages.update_all(["author_id = ?", new_user.id], ["author_id = ?", user_id])
-    if existing = conversation.conversation_participants.find_by_user_id(new_user.id)
-      existing.update_attribute(:workflow_state, workflow_state) if unread? || existing.archived?
-      destroy
-    else
-      update_attribute :user_id, new_user.id
+    self.class.send :with_exclusive_scope do
+      conversation.conversation_messages.update_all(["author_id = ?", new_user.id], ["author_id = ?", user_id])
+      if existing = conversation.conversation_participants.find_by_user_id(new_user.id)
+        existing.update_attribute(:workflow_state, workflow_state) if unread? || existing.archived?
+        destroy
+      else
+        update_attribute :user_id, new_user.id
+      end
+      conversation.regenerate_private_hash! if private?
     end
-    conversation.regenerate_private_hash! if private?
   end
 
   attr_writer :last_message
diff --git a/db/migrate/20120404230916_fix_user_merge_conversations2.rb b/db/migrate/20120404230916_fix_user_merge_conversations2.rb
new file mode 100644
index 00000000000..312652a7184
--- /dev/null
+++ b/db/migrate/20120404230916_fix_user_merge_conversations2.rb
@@ -0,0 +1,38 @@
+class FixUserMergeConversations2 < ActiveRecord::Migration
+  tag :postdeploy
+
+  def self.up
+    # basically we are re-running lines 408-410 and 417-419 of
+    # Conversation#merge_into (plus replicating some surrounding setup logic)
+    # for any private conversations that were merged into existing private
+    # conversations since 57d3a82.
+    # the previous merging was done incorrectly due to a scoping issue
+    
+    # there are only about 100 that need to be fixed, so we just load them all
+    convos = ConversationParticipant.find(:all, :conditions => "NOT EXISTS (SELECT 1 FROM conversations WHERE id = conversation_id)")
+    convos.group_by(&:conversation_id).each do |conversation_id, cps|
+      private_hash = Conversation.private_hash_for(cps.map(&:user_id))
+      if target = Conversation.find_by_private_hash(private_hash)
+        new_participants = target.conversation_participants.inject({}){ |h,p| h[p.user_id] = p; h }
+        cps.each do |cp|
+          if new_cp = new_participants[cp.user_id]
+            new_cp.update_attribute(:workflow_state, cp.workflow_state) if cp.unread? || new_cp.archived?
+            cp.conversation_message_participants.update_all(["conversation_participant_id = ?", new_cp.id])
+            cp.destroy
+          else
+            $stderr.puts "couldn't find a target ConversationParticipant for id #{cp.id}"
+          end
+        end
+        target.conversation_participants(true).each do |cp|
+          cp.update_cached_data! :recalculate_count => true, :set_last_message_at => false, :regenerate_tags => false
+        end
+      else
+        $stderr.puts "couldn't find a target Conversation for hash #{private_hash}"
+      end
+    end
+  end
+
+  def self.down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end
diff --git a/spec/migrations/fix_user_merge_conversations2.rb b/spec/migrations/fix_user_merge_conversations2.rb
new file mode 100644
index 00000000000..03bbfe74e28
--- /dev/null
+++ b/spec/migrations/fix_user_merge_conversations2.rb
@@ -0,0 +1,77 @@
+#
+# Copyright (C) 2011 Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+
+require File.expand_path(File.dirname(__FILE__) + '/../spec_helper.rb')
+require 'db/migrate/20120404230916_fix_user_merge_conversations2.rb'
+
+describe FixUserMergeConversations2 do
+  describe "up" do
+    it "should work" do
+      u1 = user
+      u2 = user
+      u3 = user
+
+      # set up borked conversation that is partially merged...
+      # conversation deleted, cp's and cmps orphaned,
+      # and cm on the target conversation
+      borked = Conversation.initiate([u1.id, u2.id], true)
+      borked_cps = borked.conversation_participants.all
+      borked_cmps = borked_cps.map(&:conversation_message_participants).flatten
+      m1 = borked.add_message(u1, "test")
+      Conversation.delete_all(:id => borked.id) # bypass callbacks
+
+      correct = Conversation.initiate([u1.id, u2.id], true)
+      m2 = correct.add_message(u1, "test2")
+      correct.conversation_participants.each { |cp| cp.update_attribute :workflow_state, 'archived'}
+
+      # put it the message on the correct conversation
+      m1.update_attribute :conversation_id, correct.id
+
+      unrelated = Conversation.initiate([u1.id, u3.id], true)
+      unrelated.add_message(u1, "test3")
+
+      FixUserMergeConversations2.up
+
+      correct.reload
+      unrelated.reload
+
+      # these are gone for reals now
+      lambda { borked.reload }.should raise_error
+      borked_cps.each { |cp| lambda { cp.reload }.should raise_error }
+
+      # these are moved to the right place
+      borked_cmps.each do |cmp|
+        lambda { cmp.reload }.should_not raise_error
+        cmp.conversation_participant.conversation.should eql correct
+      end
+      correct.conversation_participants.each do |cp|
+        cp.messages.size.should eql 2
+        cp.message_count.should eql 2
+      end
+      # got bumped out of archived by the merged/deleted ones
+      correct.conversation_participants.default.size.should eql 2
+      correct.conversation_participants.unread.size.should eql 1
+
+      # no changes here
+      unrelated.conversation_participants.each do |cp|
+        cp.messages.size.should eql 1
+        cp.message_count.should eql 1
+      end
+    end
+  end
+end
diff --git a/spec/models/conversation_participant_spec.rb b/spec/models/conversation_participant_spec.rb
index 92675f808f2..ab9b1e36464 100644
--- a/spec/models/conversation_participant_spec.rb
+++ b/spec/models/conversation_participant_spec.rb
@@ -338,5 +338,31 @@ describe ConversationParticipant do
       @user1.reload.unread_conversations_count.should eql 0
       @user2.reload.unread_conversations_count.should eql 1
     end
+
+    it "should not be adversely affected by an outer scope" do
+      other_guy = user
+      c = @user1.initiate_conversation([other_guy.id])
+      c.add_message("hello")
+      c.update_attribute(:workflow_state, 'unread')
+      c2 = @user2.initiate_conversation([other_guy.id])
+      c2.add_message("hola")
+
+      c.reload
+      ConversationParticipant.send :with_scope, :find => {:conditions => ["user_id = ?", @user1.id]} do
+        c.move_to_user @user2
+      end
+  
+      lambda{ c.reload }.should raise_error # deleted
+      lambda{ Conversation.find(c.conversation_id) }.should raise_error # deleted
+
+      c2.reload.messages.size.should eql 2
+      c2.messages.map(&:author_id).should eql [@user2.id, @user2.id]
+      c2.message_count.should eql 2
+      c2.user_id.should eql @user2.id
+      c2.conversation.participants.size.should eql 2
+      @user1.reload.unread_conversations_count.should eql 0
+      @user2.reload.unread_conversations_count.should eql 1
+      other_guy.reload.unread_conversations_count.should eql 1
+    end
   end
 end