unindexed_foreign_keys: handle foreign keys on primary keys

This commit fixes an edge case where a foreign key is defined on the
primary key. Primary keys are already indexed and have to be unique,
so it's not necessary to report them.

lib/active_record_doctor/detectors/unindexed_foreign_keys.rb

@@ -30,6 +30,7 @@ def detect
             next unless named_like_foreign_key?(column) || foreign_key?(table, column)
             next if indexed?(table, column)
             next if indexed_as_polymorphic?(table, column)
+            next if connection.primary_key(table) == column.name
 
             type_column_name = type_column_name(column)
 

test/active_record_doctor/detectors/unindexed_foreign_keys_test.rb

@@ -61,6 +61,14 @@ def test_indexed_foreign_key_is_not_reported
     refute_problems
   end
 
+  def test_foreign_key_is_a_primary_key_not_reported
+    Context.create_table(:parents)
+    Context.create_table(:children, primary_key: :parent_id)
+    ActiveRecord::Base.connection.add_foreign_key(:children, :parents)
+
+    refute_problems
+  end
+
   def test_config_ignore_tables
     skip("MySQL always indexes foreign keys") if mysql?