title: Efficient way to bulk insert / update using typeORM
body: <p>How it's possible to insert pre-created objects into relation with an object using TypeORM?
  As I know the non-efficient is this(many to many relations):</p>
  
  <pre><code> const tags = await Tag.findByIds(tagIds);
   const todo = await Todo.findOne(todoId);
   todo.tags = tags;
   return await Todo.save(todo)
  </code></pre>
  
  <p>Also <code>.add()</code> property of <code>RelationalQueryBuilder</code> takes single object or Id as is mentioned in <a href="https://github.com/typeorm/typeorm/blob/master/docs/relational-query-builder.md" rel="nofollow noreferrer">docs</a></p>
  
  <pre><code>await getConnection()
      .createQueryBuilder()
      .relation(Post, "categories")
      .of(1)
      .add(3);
  </code></pre>
  
  <p>So what is the efficient way to bulk insert?</p>