uniqueness validation raises error for persisted record without pk.

Closes #21304.

While we can validate uniqueness for record without primary key on
creation, there is no way to exclude the current record when
updating. (The update itself will need a primary key to work correctly).

2 files changed, 9 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/errors.rb b/activerecord/lib/active_record/errors.rb
index d589620f8a..718f04871d 100644
--- a/activerecord/lib/active_record/errors.rb
+++ b/activerecord/lib/active_record/errors.rb
@@ -218,11 +218,12 @@ module ActiveRecord
   class UnknownPrimaryKey < ActiveRecordError
     attr_reader :model
 
-    def initialize(model)
-      super("Unknown primary key for table #{model.table_name} in model #{model}.")
+    def initialize(model, description = nil)
+      message = "Unknown primary key for table #{model.table_name} in model #{model}."
+      message += "\n#{description}" if description
+      super(message)
       @model = model
     end
-
   end
 
   # Raised when a relation cannot be mutated because it's already loaded.
diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index 32d17a1392..5706bbd903 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -18,7 +18,11 @@ module ActiveRecord
 
         relation = build_relation(finder_class, table, attribute, value)
         if record.persisted? && finder_class.primary_key.to_s != attribute.to_s
-          relation = relation.where.not(finder_class.primary_key => record.id)
+          if finder_class.primary_key
+            relation = relation.where.not(finder_class.primary_key => record.id)
+          else
+            raise UnknownPrimaryKey.new(finder_class, "Can not validate uniqueness for persisted record without primary key.")
+          end
         end
         relation = scope_relation(record, table, relation)
         relation = relation.merge(options[:conditions]) if options[:conditions]