1 files changed, 30 insertions, 1 deletions
diff --git a/lib/gitlab/sql/set_operator.rb b/lib/gitlab/sql/set_operator.rb
index 59a808eafa9..18275da3ef0 100644
--- a/lib/gitlab/sql/set_operator.rb
+++ b/lib/gitlab/sql/set_operator.rb
@@ -19,6 +19,7 @@ module Gitlab
     #     Project.where("id IN (#{sql})")
     class SetOperator
       def initialize(relations, remove_duplicates: true, remove_order: true)
+        verify_select_values!(relations) if Rails.env.test? || Rails.env.development?
         @relations = relations
         @remove_duplicates = remove_duplicates
         @remove_order = remove_order
@@ -33,7 +34,7 @@ module Gitlab
         # aren't incremented properly when joining relations together this way.
         # By using "unprepared_statements" we remove the usage of placeholders
         # (thus fixing this problem), at a slight performance cost.
-        fragments = ActiveRecord::Base.connection.unprepared_statement do
+        fragments = ApplicationRecord.connection.unprepared_statement do
           relations.map do |rel|
             remove_order ? rel.reorder(nil).to_sql : rel.to_sql
           end.reject(&:blank?)
@@ -54,6 +55,34 @@ module Gitlab
       private
 
       attr_reader :relations, :remove_duplicates, :remove_order
+
+      def verify_select_values!(relations)
+        all_select_values = relations.map do |relation|
+          if relation.respond_to?(:select_values)
+            relation.select_values
+          else
+            relation.projections # Handle Arel based subqueries
+          end
+        end
+
+        unless all_select_values.map(&:size).uniq.one?
+          relation_select_sizes = all_select_values.map.with_index do |select_values, index|
+            if select_values.empty?
+              "Relation ##{index}: *, all columns"
+            else
+              "Relation ##{index}: #{select_values.size} select values"
+            end
+          end
+
+          exception_text = <<~EOF
+          Relations with uneven select values were passed. The UNION query could break when the underlying table changes (add or remove columns).
+
+          #{relation_select_sizes.join("\n")}
+          EOF
+
+          raise(exception_text)
+        end
+      end
     end
   end
 end