4 files changed, 39 insertions, 4 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index baaa38466b..0841b2f679 100644
--- a/activerecord/CHANGELOG.md
+++ b/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:
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]
diff --git a/activerecord/test/cases/validations/uniqueness_validation_test.rb b/activerecord/test/cases/validations/uniqueness_validation_test.rb
index ceca2c8366..7502a55391 100644
--- a/activerecord/test/cases/validations/uniqueness_validation_test.rb
+++ b/activerecord/test/cases/validations/uniqueness_validation_test.rb
@@ -4,6 +4,7 @@ require 'models/reply'
 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 @@ class UniquenessValidationTest < ActiveRecord::TestCase
     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