'use strict';

module.exports = {
  async up(queryInterface, Sequelize) {
    await queryInterface.createTable('course_instructors', {
      id:           { type: Sequelize.BIGINT, primaryKey: true, autoIncrement: true },
      course_id:    { type: Sequelize.BIGINT, allowNull: false, references: { model: 'courses', key: 'course_id' }, onDelete: 'CASCADE' },
      user_id:      { type: Sequelize.BIGINT, allowNull: true, references: { model: 'users', key: 'user_id' }, onDelete: 'CASCADE' },
      order_index:  { type: Sequelize.BIGINT, allowNull: false, defaultValue: 0 },
      display_name: { type: Sequelize.STRING(255), allowNull: false, defaultValue: '' },
      created_by:   { type: Sequelize.BIGINT, allowNull: true },
      created_at:   { type: Sequelize.DATE, allowNull: false, defaultValue: Sequelize.NOW },
      updated_at:   { type: Sequelize.DATE, allowNull: false, defaultValue: Sequelize.NOW },
    });

    await queryInterface.addConstraint('course_instructors', {
      fields: ['course_id', 'user_id'],
      type: 'unique',
      name: 'course_instructors_course_id_user_id_key',
    });

    await queryInterface.addIndex('course_instructors', ['course_id'], { name: 'idx_course_instructors_course' });

    // Partial unique index — same columns as the constraint above, scoped to
    // non-null user_id. Raw SQL since queryInterface addIndex's `where`
    // support is unreliable for IS NOT NULL across dialects.
    await queryInterface.sequelize.query(`
      CREATE UNIQUE INDEX idx_course_instructors_user
      ON course_instructors (course_id, user_id)
      WHERE user_id IS NOT NULL;
    `);
  },

  async down(queryInterface) {
    await queryInterface.dropTable('course_instructors');
  },
};