ActiveRecord::SchemaMigration has no primary key.

Before this patch, using `ActiveRecord::Base.primary_key_prefix_type`
with `:table_name_with_underscore` would change the `SchemaMigration` model
to have a primary key. This resulted in broken queries for PG because it tried
to return the inserted PK (which does not exist).

Closes #15051.
Closes #15419.

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   `ActiveRecord::SchemaMigration` has no primary key regardless of the
+    `primary_key_prefix_type` configuration.
+
+    Fixes #15051.
+
+    *Yves Senn*
+
 *   `rake db:migrate:status` works with legacy migration numbers like `00018_xyz.rb`.
 
     Fixes #15538.

activerecord/lib/active_record/schema_migration.rb

@@ -5,6 +5,9 @@ require 'active_record/base'
 module ActiveRecord
   class SchemaMigration < ActiveRecord::Base
     class << self
+      def primary_key
+        nil
+      end
 
       def table_name
         "#{table_name_prefix}#{ActiveRecord::Base.schema_migrations_table_name}#{table_name_suffix}"

activerecord/test/cases/ar_schema_test.rb

@@ -17,6 +17,20 @@ if ActiveRecord::Base.connection.supports_migrations?
       ActiveRecord::SchemaMigration.delete_all rescue nil
     end
 
+    def test_has_no_primary_key
+      old_primary_key_prefix_type = ActiveRecord::Base.primary_key_prefix_type
+      ActiveRecord::Base.primary_key_prefix_type = :table_name_with_underscore
+      assert_nil ActiveRecord::SchemaMigration.primary_key
+
+      ActiveRecord::SchemaMigration.create_table
+      assert_difference "ActiveRecord::SchemaMigration.count", 1 do
+        ActiveRecord::SchemaMigration.create version: 12
+      end
+    ensure
+      ActiveRecord::SchemaMigration.drop_table
+      ActiveRecord::Base.primary_key_prefix_type = old_primary_key_prefix_type
+    end
+
     def test_schema_define
       ActiveRecord::Schema.define(:version => 7) do
         create_table :fruits do |t|