BUGFIX/FEATURE: store topic changes in post revisions

History + edit notifications for title and category changes

app/models/post_revision.rb

@@ -1,8 +1,96 @@
+require_dependency "discourse_diff"
+
 class PostRevision < ActiveRecord::Base
   belongs_to :post
   belongs_to :user
 
   serialize :modifications, Hash
+
+  def body_changes
+    changes_for("cooked", "raw")
+  end
+
+  def category_changes
+    {
+      previous_category_id: previous("category_id"),
+      current_category_id: current("category_id"),
+    }
+  end
+
+  def title_changes
+    changes_for("title", nil, true)
+  end
+
+  def changes_for(name, markdown=nil, wrap=false)
+    prev = previous(name)
+    cur = current(name)
+
+    if wrap
+      prev = "<div>#{CGI::escapeHTML(prev)}</div>"
+      cur = "<div>#{CGI::escapeHTML(cur)}</div>"
+    end
+
+    diff = DiscourseDiff.new(prev, cur)
+
+    result = {
+      inline: diff.inline_html,
+      side_by_side: diff.side_by_side_html
+    }
+
+    if markdown
+      diff = DiscourseDiff.new(previous(markdown), current(markdown))
+      result[:side_by_side_markdown] = diff.side_by_side_markdown
+    end
+
+    result
+  end
+
+  def previous(field)
+    lookup_with_fallback(field, 0)
+  end
+
+  def current(field)
+    lookup_with_fallback(field, 1)
+  end
+
+  def previous_revisions
+    @previous_revs ||=
+      PostRevision.where("post_id = ? AND number < ?",
+                              post_id,        number
+                      )
+                .order("number desc")
+                .to_a
+  end
+
+  def has_topic_data?
+    post && post.post_number == 1
+  end
+
+  def lookup_with_fallback(field, index)
+
+    unless val = lookup(field, index)
+      previous_revisions.each do |v|
+        break if val = v.lookup(field, 1)
+      end
+    end
+
+    unless val
+      if ["cooked","raw"].include?(field)
+        val = post.send(field)
+      else
+        val = post.topic.send(field)
+      end
+    end
+
+    val
+  end
+
+  def lookup(field, index)
+    if mod = modifications[field]
+      mod[index]
+    end
+  end
+
 end
 
 # == Schema Information