Fix versions migration for postgres 12

lib/sequent/migrations/versions.rb

@@ -11,8 +11,10 @@ class Versions < Sequent::ApplicationRecord
       def self.migration_sql
         <<~SQL.chomp
           CREATE TABLE IF NOT EXISTS #{table_name} (version integer NOT NULL, CONSTRAINT version_pk PRIMARY KEY(version));
+          ALTER TABLE #{table_name} drop constraint if exists only_one_running;
           ALTER TABLE #{table_name} ADD COLUMN IF NOT EXISTS status INTEGER DEFAULT NULL CONSTRAINT only_one_running CHECK (status in (1,2,3));
-          CREATE UNIQUE INDEX IF NOT EXISTS single_migration_running ON #{table_name} (status);
+          DROP INDEX IF EXISTS single_migration_running;
+          CREATE UNIQUE INDEX single_migration_running ON #{table_name} ((status * 0)) where status is not null;
         SQL
       end
 

spec/lib/sequent/migrations/view_schema_spec.rb

@@ -59,6 +59,17 @@
 
       expect(Sequent::ApplicationRecord.connection).to have_schema(view_schema)
     end
+
+    it 'can not insert two versions with a status' do
+      migrator.create_view_schema_if_not_exists
+      migrator.create_view_schema_if_not_exists
+
+      Sequent::Migrations::Versions.create!(version: 1, status: nil)
+      Sequent::Migrations::Versions.create!(version: 2, status: 1)
+      expect do
+        Sequent::Migrations::Versions.create!(version: 3, status: 2)
+      end.to raise_error(ActiveRecord::RecordNotUnique)
+    end
   end
 
   context '#create_view_tables' do