1 files changed, 4 insertions, 2 deletions
diff --git a/activerecord/lib/active_record/insert_all.rb b/activerecord/lib/active_record/insert_all.rb
index f6577dcbc4..3863b6bfea 100644
--- a/activerecord/lib/active_record/insert_all.rb
+++ b/activerecord/lib/active_record/insert_all.rb
@@ -14,7 +14,7 @@ module ActiveRecord
       @returning = (connection.supports_insert_returning? ? primary_keys : false) if @returning.nil?
       @returning = false if @returning == []
 
-      @unique_by = find_unique_index_for(unique_by) if unique_by
+      @unique_by = find_unique_index_for(unique_by || model.primary_key)
       @on_duplicate = :skip if @on_duplicate == :update && updatable_columns.empty?
 
       ensure_valid_options_for_connection!
@@ -32,7 +32,7 @@ module ActiveRecord
     end
 
     def primary_keys
-      Array(model.primary_key)
+      Array(connection.schema_cache.primary_keys(model.table_name))
     end
 
 
@@ -61,6 +61,8 @@ module ActiveRecord
 
         if index = unique_indexes.find { |i| match.include?(i.name) || i.columns == match }
           index
+        elsif match == primary_keys
+          nil
         else
           raise ArgumentError, "No unique index found for #{unique_by}"
         end