Sequelize bug

bug.js

const Sequelize = require('sequelize');
const config = require('./config')();

const {DataTypes} = Sequelize;

const db = new Sequelize({
	database: config.postgres.database,
	username: config.postgres.user,
	password: config.postgres.password,
	host: config.postgres.host,
	port: config.postgres.port,
	dialect: 'postgres',
	logging: false
});

db.parents = db.define('Parent', {
	_id: {
		type: DataTypes.INTEGER,
		primaryKey: true,
		autoIncrement: true
	}
}, {
	timestamps: false,
	tableName: 'parents'
});

db.somethings = db.define('Something', {
	_id: {type: DataTypes.INTEGER, primaryKey: true, autoIncrement: true}
}, {
	timestamps: false,
	tableName: 'somethings'
});

db.children = db.define('Child', {
	_id: {type: DataTypes.INTEGER, primaryKey: true, autoIncrement: true},
	parentId: DataTypes.INTEGER,
	somethingId: DataTypes.INTEGER
}, {
	timestamps: false,
	tableName: 'children'
});

db.parents.hasMany(db.children, {
	as: 'children',
	foreignKey: 'parentId'
});

db.somethings.hasMany(db.children, {
	as: 'children',
	foreignKey: 'somethingId'
});

db.children.belongsTo(db.parents, {
	as: 'parent',
	foreignKey: 'parentId',
	targetKey: '_id'
});

db.children.belongsTo(db.somethings, {
	as: 'something',
	foreignKey: 'somethingId'
});

(async () => {
	try {
		await db.transaction(async (transaction) => {
			await db.parents.truncate({transaction});
			await db.somethings.truncate({transaction});
			await db.children.truncate({transaction});

			const {_id: parentId} = await db.parents.create({}, {transaction});
			const {_id: somethingId} = await db.somethings.create({}, {transaction});

			await db.children.bulkCreate(
				[{parentId, somethingId}, {parentId, somethingId}],
				{transaction}
			);

			const parents = await db.parents.findAll({
				include: [{
					model: db.children,
					as: 'children',
					required: true,
					include: [{
						model: db.somethings,
						as: 'something',
						required: true
					}]
				}],
				limit: 5,
				transaction,
				logging: console.log
			});

			console.log(
				require('util').inspect(
					parents.map((parent) => parent.toJSON()),
					false,
					null
				)
			);
		});
	} catch (err) {
		console.log(err.stack);
	}
})();