Properly allow uniqueness validations on primary keys.

This is an alternate implementation of #20966.

[Sean Griffin & presskey]

3 files changed, 28 insertions, 1 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 99d9101347..4cf54ffeb1 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Properly allow uniqueness validations on primary keys.
+
+    Fixes #20966.
+
+    *Sean Griffin & presskey*
+
 *   Don't raise an error if an association failed to destroy when `destroy` was
     called on the parent (as opposed to `destroy!`).
 
diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index 5106f4e127..32d17a1392 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -17,7 +17,9 @@ module ActiveRecord
         value = map_enum_attribute(finder_class, attribute, value)
 
         relation = build_relation(finder_class, table, attribute, value)
-        relation = relation.where.not(finder_class.primary_key => record.id) if record.persisted?
+        if record.persisted? && finder_class.primary_key.to_s != attribute.to_s
+          relation = relation.where.not(finder_class.primary_key => record.id)
+        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 2608c84be2..ceca2c8366 100644
--- a/activerecord/test/cases/validations/uniqueness_validation_test.rb
+++ b/activerecord/test/cases/validations/uniqueness_validation_test.rb
@@ -427,4 +427,23 @@ class UniquenessValidationTest < ActiveRecord::TestCase
     assert reply.valid?
     assert topic.valid?
   end
+
+  def test_validate_uniqueness_of_custom_primary_key
+    klass = Class.new(ActiveRecord::Base) do
+      self.table_name = "keyboards"
+      self.primary_key = :key_number
+
+      validates_uniqueness_of :key_number
+
+      def self.name
+        "Keyboard"
+      end
+    end
+
+    klass.create!(key_number: 10)
+    key2 = klass.create!(key_number: 11)
+
+    key2.key_number = 10
+    assert_not key2.valid?
+  end
 end