search: fix order:custom metatag.

Fix order:custom not working. Also change order:custom to return no
posts under the following error conditions:

* {{order:custom}} (id metatag isn't present)
* {{id:42 order:custom}} (id metatag isn't a list)
* {{id:>42 order:custom}} (id metatag isn't a list)
* {{id:1,2 id:2,3 order:custom}} (id metatag is present twice)

app/logical/post_query_builder.rb

@@ -607,8 +607,8 @@ class PostQueryBuilder
       relation = relation.where("posts.image_width IS NOT NULL and posts.image_height IS NOT NULL")
     end
 
-    if q[:order] == "custom" && q[:post_id].present? && q[:post_id][0] == :in
-      relation = relation.find_ordered(q[:post_id][1])
+    if q[:order] == "custom"
+      relation = search_order_custom(relation, q[:id])
     else
       relation = search_order(relation, q[:order])
     end
@@ -739,6 +739,15 @@ class PostQueryBuilder
     relation
   end
 
+  def search_order_custom(relation, id_metatags)
+    return relation.none unless id_metatags.present? && id_metatags.size == 1
+
+    operator, ids = parse_range(id_metatags.first, :integer)
+    return relation.none unless operator == :in
+
+    relation.find_ordered(ids)
+  end
+
   concerning :ParseMethods do
     def scan_query
       terms = []

test/unit/post_query_builder_test.rb

@@ -809,6 +809,20 @@ class PostQueryBuilderTest < ActiveSupport::TestCase
       assert_tag_match([post2, post1, post3], "order:comment_bumped_asc")
     end
 
+    should "return posts for order:custom" do
+      p1 = create(:post)
+      p2 = create(:post)
+      p3 = create(:post)
+
+      as(create(:gold_user)) do
+        assert_tag_match([p2, p1, p3], "id:#{p2.id},#{p1.id},#{p3.id} order:custom")
+        assert_tag_match([], "id:#{p1.id} order:custom")
+        assert_tag_match([], "id:>0 order:custom")
+        assert_tag_match([], "id:1,2 id:2,3 order:custom")
+        assert_tag_match([], "order:custom")
+      end
+    end
+
     should "return posts for a filesize search" do
       post = create(:post, file_size: 1.megabyte)