`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.

1 files changed, 3 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/schema_migration.rb b/activerecord/lib/active_record/schema_migration.rb
index 5e484ded75..3a004d58c9 100644
--- a/activerecord/lib/active_record/schema_migration.rb
+++ b/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}"