FIX: ensure double flagging is impossible due to db constraint

db/migrate/20140904055702_correct_post_action_index.rb

@@ -0,0 +1,47 @@
+class CorrectPostActionIndex < ActiveRecord::Migration
+
+  def up
+
+    # NOTE: not stored in the table yet, is_flag contains notify_user
+    #  hardcoding, long term we can store all metadata and join for index
+    # This means that if we muck with flag types we need to redo this index
+    #  flag types have been stable for a while so we should be ok
+    #
+    # Another solution is hoisting an extra boolean into the post action table
+    #
+    # {:off_topic=>3, :inappropriate=>4, :notify_moderators=>7, :spam=>8}
+    flag_ids = "3,4,7,8"
+
+   x = execute "DELETE FROM post_actions pa
+                    USING post_actions pa2
+             WHERE pa.post_action_type_id IN (#{flag_ids}) AND
+                   pa2.post_action_type_id IN (#{flag_ids}) AND
+                   pa.deleted_at IS NULL AND
+                   pa2.deleted_at IS NULL AND
+                   pa.disagreed_at IS NULL AND
+                   pa2.disagreed_at IS NULL AND
+                   pa.deferred_at IS NULL AND
+                   pa2.deferred_at IS NULL AND
+                   pa.id < pa2.id AND
+                   pa.user_id = pa2.user_id AND
+                   pa.post_id = pa2.post_id AND
+                   pa.targets_topic = pa2.targets_topic"
+
+    puts
+    puts ">> DELETED #{x.cmd_tuples} invalid rows from post_actions"
+    puts
+
+    add_index :post_actions,
+                ["user_id", "post_id", "targets_topic"],
+                name: "idx_unique_flags",
+                unique: true,
+                where: "deleted_at IS NULL AND
+                        disagreed_at IS NULL AND
+                        deferred_at IS NULL AND
+                        post_action_type_id IN (#{flag_ids})"
+  end
+
+  def down
+    remove_index "post_actions", name: "idx_unique_flags"
+  end
+end