It turns out comments weren't importing because I didn't write the code to make them import.

lib/data_conversion/import_to_mysql.rb

@@ -34,6 +34,7 @@ module DataConversion
       process_raw_requests
       process_raw_profiles
       process_raw_posts
+      process_raw_comments
       process_raw_post_visibilities
       process_raw_notifications
     end
@@ -227,6 +228,26 @@ module DataConversion
       SQL
       log "Imported #{Service.count} services."
     end
+    def process_raw_comments
+      log "Importing comments to main table..."
+      Comment.connection.execute <<-SQL
+        INSERT INTO comments
+        SELECT mongo_comments.id,
+               mongo_comments.text,
+               posts.id,
+               people.id,
+               mongo_comments.guid,
+               mongo_comments.creator_signature,
+               mongo_comments.post_creator_signature,
+               mongo_comments.youtube_titles,
+               mongo_comments.created_at,
+               mongo_comments.updated_at,
+               mongo_comments.mongo_id
+          FROM mongo_comments INNER JOIN (posts, people)
+            ON (posts.mongo_id = mongo_comments.post_mongo_id AND people.mongo_id = mongo_comments.person_mongo_id)
+      SQL
+      log "Imported #{Comment.count} comments."
+    end
     def process_raw_people
       log "Importing people to main table..."
       Person.connection.execute <<-SQL

spec/lib/data_conversion/import_to_mysql_spec.rb

@@ -367,6 +367,36 @@ describe DataConversion::ImportToMysql do
         post.updated_at.should == mongo_post.updated_at
       end
     end
+    describe "comments" do
+      before do
+        import_and_process("users")
+        import_and_process("people")
+        import_and_process("posts")
+        copy_fixture_for("comments")
+        @migrator.import_raw_comments
+      end
+
+      it "imports data into the mongo_comments table" do
+        Mongo::Comment.count.should == 2
+        Comment.count.should == 0
+        @migrator.process_raw_comments
+        Comment.count.should == 2
+      end
+
+      it "processes all the columns" do
+        @migrator.process_raw_comments
+        comment = Comment.first
+        comment.mongo_id.should == "4d2b6ebfcc8cb43cc200002b"
+        comment.text.should == "Hey me!"
+        comment.youtube_titles.should be_nil
+      end
+      it 'sets the relations' do
+        @migrator.process_raw_comments
+        comment = Comment.first
+        comment.post_id.should == Post.where(:mongo_id => "4d2b6ebecc8cb43cc2000029").first.id
+        comment.person_id.should == Person.where(:mongo_id => "4d2b6eb7cc8cb43cc2000017").first.id
+      end
+    end
     describe "notifications" do
       before do
         import_and_process("users")