diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 6fa99c6d94761..c779734f0704b 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -444,11 +444,11 @@ def construct_relation_for_exists(conditions)
           relation = except(:select, :distinct, :order)._select!(ONE_AS_ONE).limit!(1)
         end
 
-        case conditions
-        when Array, Hash
+        if conditions.is_a?(Array) && conditions.first.is_a?(String) || conditions.is_a?(Hash)
           relation.where!(conditions) unless conditions.empty?
         else
-          relation.where!(primary_key => conditions) unless conditions == :none
+          conditions = [conditions] if klass.composite_primary_key?
+          relation.where!(primary_key => conditions) unless conditions == :none || conditions.empty?
         end
 
         relation