create-blog-post-with-new-and-existing-tags.js

const { tags, ...blogPost } = {
  "title": "My second blog post",
  "body": "...",
  "tags": [
    { "id": 1, "text": "sequelize" }, // the existing tag has an "id"
    { "text": "database" } // the new tag does not have an "id"
   ]
};

const newPost = db.create('blog_posts', blogPost);
const existingTags = tags.filter(tag => tag.id !== undefined);
const newTags = db.createMany('tags', tags.filter(tag => tag.id === undefined));
const allTags = existingTags.concat(newTags);
const blogPostTags = allTags.map(tag => {
  return { 
    tag_id: tag.id, 
    blog_post_id: newPost.id 
  };
});

db.createMany('blog_post_tags', blogPostTags);