Fail when issuable_meta_data is called on an unlimited collectionfix-issues-api-list-performance

This method can be called with an array, or a relation:

1. Arrays always have a limited amount of values, so that's fine.
2. If the relation does not have a limit value applied, then we will load every
   single object in that collection, and prevent N+1 queries for the metadata
   for that. But that's wrong, because we should never call this without an
   explicit limit set. So we raise in that case, and this commit will see which
   specs fail.

The only failing specs here were the issues API specs, and the specs for
IssuableMetadata itself, and both have been addressed.

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/gitlab/issuable_metadata.rb b/lib/gitlab/issuable_metadata.rb
index 977c05910d3..0c9de72329c 100644
--- a/lib/gitlab/issuable_metadata.rb
+++ b/lib/gitlab/issuable_metadata.rb
@@ -1,6 +1,14 @@
 module Gitlab
   module IssuableMetadata
     def issuable_meta_data(issuable_collection, collection_type)
+      # ActiveRecord uses Object#extend for null relations.
+      if !(issuable_collection.singleton_class < ActiveRecord::NullRelation) &&
+          issuable_collection.respond_to?(:limit_value) &&
+          issuable_collection.limit_value.nil?
+
+        raise 'Collection must have a limit applied for preloading meta-data'
+      end
+
       # map has to be used here since using pluck or select will
       # throw an error when ordering issuables by priority which inserts
       # a new order into the collection.