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).

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Uniqueness validator raises descriptive error when running on a persisted
+    record without primary key.
+
+    Closes #21304.
+
+    *Yves Senn*
+
 *   Add a native JSON data type support in MySQL.
 
     Example:

activerecord/lib/active_record/errors.rb

@@ -218,11 +218,12 @@ def initialize(errors)
   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.

activerecord/lib/active_record/validations/uniqueness.rb

@@ -18,7 +18,11 @@ def validate_each(record, attribute, value)
 
         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]

activerecord/test/cases/validations/uniqueness_validation_test.rb

@@ -4,6 +4,7 @@
 require 'models/warehouse_thing'
 require 'models/guid'
 require 'models/event'
+require 'models/dashboard'
 
 class Wizard < ActiveRecord::Base
   self.abstract_class = true
@@ -446,4 +447,26 @@ def self.name
     key2.key_number = 10
     assert_not key2.valid?
   end
+
+  def test_validate_uniqueness_without_primary_key
+    klass = Class.new(ActiveRecord::Base) do
+      self.table_name = "dashboards"
+
+      validates_uniqueness_of :dashboard_id
+
+      def self.name; "Dashboard" end
+    end
+
+    abc = klass.create!(dashboard_id: "abc")
+    assert klass.new(dashboard_id: "xyz").valid?
+    assert_not klass.new(dashboard_id: "abc").valid?
+
+    abc.dashboard_id = "def"
+
+    e = assert_raises ActiveRecord::UnknownPrimaryKey do
+      abc.save!
+    end
+    assert_match(/\AUnknown primary key for table dashboards in model/, e.message)
+    assert_match(/Can not validate uniqueness for persisted record without primary key.\z/, e.message)
+  end
 end