No special messages at the moment. Just have a good day and stay hydrated!
Close

Junction Model Pattern: Many-to-Many - Sequelize

Apr 30th, 2019 / 4 min read / Share / Edit on GitHub
Many-to-many is a common modeling relationship between two entities. Here's one way to handle it with the Sequelize ORM.

Let's assume you're building a blog with Sequelize.

On your blog, you can create a bunch of Posts. As a way to describe your post, it can belong to many different Genres.

This blog post for example, might have the Sequelize, Orm, and Web Development tag.

So far, we have two models: Post and Genre.

// Post.js
module.exports = function(sequelize, DataTypes) {
  const Post =  sequelize.define('post', {
    post_id: {
      type: DataTypes.INTEGER(11),
      allowNull: false,
      autoIncrement: true,
      primaryKey: true
    },
    text: {
      type: DataTypes.TEXT,
      allowNull: false
    }
  }, {
    timestamps: true,
    underscored: true,
    tableName: 'post',
  });

  return Post;
};
// Genre.js
module.exports = function(sequelize, DataTypes) {
  const Genre =  sequelize.define('genre', {
    genre_id: {
      type: DataTypes.INTEGER(11),
      allowNull: false,
      autoIncrement: true,
      primaryKey: true
    },
    name: {
      type: DataTypes.STRING(60),
      allowNull: false,
      unique: true
    }
  }, {
    timestamps: true,
    underscored: true,
    tableName: 'genre',
  });

  return Genre;
};

Great. Now how do we model the many-to-many relationship between them?

Modeling the junction table

We need to create a junction table to store the relationship between the two.

In order to model it, we'll end up with three tables instead of just two.

I know that there's a way to get Sequelize to create this for you simply by using the correct association methods, but I like to be explicit about it.

Let's go ahead and create the additional model now.

// tagPostGenre.js
module.exports = function(sequelize, DataTypes) {
  const TagPostGenre = sequelize.define('tag_post_genre', {
    tag_post_genre_id: {
      type: DataTypes.UUID,
      defaultValue: DataTypes.INTEGER(11),
      primaryKey: true
    },
    post_id: {
      type: DataTypes.INTEGER(11),
      primaryKey: false,
      references: {
        model: 'post',
        key: 'post_id'
      },
      onDelete: 'cascade',
      onUpdate: 'cascade',
      unique: 'unique-genre-per-post'
    },
    genre_id: {
      type: DataTypes.INTEGER(11),
      primaryKey: false,
      references: {
        model: 'genre',
        key: 'genre_id'
      },
      onDelete: 'cascade',
      onUpdate: 'cascade',
      unique: 'unique-genre-per-post'
    },
  }, {
    timestamps: true,
    underscored: true,
    tableName: 'tag_post_genre'
  });

  return TagPostGenre;
};

Also, it's not necessary to prepend the model name with "Tag-", I just like to do that because it allows me to understand at a glance that that table is a Junction table between the other two tables mentioned in the name.

Here's what's important about this so far:

  • We've created foreign key relationships to the Post and Genre model.
  • We added a composite unique constraint in order to prevent a post from adding the same genre twice.

Now that we have the model created, we have to associate them all together with Sequelize so that we can get the benefits of using the convenience methods they expose.

Update the Associations

Add the associations on TagPostGenre junction table/model.

TagPostGenre.associate = (models) => {
  TagPostGenre.belongsTo(models.Post, { foreignKey: 'post_id', targetKey: 'post_id', as: 'Post' });
  TagPostGenre.belongsTo(models.Genre, { foreignKey: 'genre_id', targetKey: 'genre_id', as: 'Genre' });
}

Add the association on Genre.

Genre.associate = (models) => {
  Genre.belongsToMany(models.Post, { as: 'PostsInGenre', through: models.TagPostGenre, foreignKey: 'genre_id'});
}

Finally, add the association on Post.

Post.associate = (models) => {
  Post.belongsToMany(models.Genre, { as: 'GenresForPost', through: models.TagPostGenre, foreignKey: 'genre_id'});
}

The as key in the second argument's config object using belongsToMany is a way to specify the alias when we're doing Eager Loading / Include Queries.

Usage

If you want to set the genres for a Post instance...

const post = await Post.findOne({ where: { post_id: 1 }});

Since we've added the associations, you should have access to the convenience methods that Sequelize adds.

In this particular case, expect to see something like setGenres() on the post instance.

const genreIds = [1,2,3];
await post.setGenres(genreIds);

const genres = await post.getGenres();
console.log(genres) // genre instances! [{}, {}, {}]

We can use the ids of the genres to set the genres for this post.

You can also retrieve all of the genres using get-"plural junction association name"(). In this case, getGenres().

You can always double check that the methods have been added by using the vscode debugger.



Sponsor

I hope this article was useful to you! Consider checking out my sponsors. I can continue to write quality articles for free because of them.

Discussion

Thoughts? Share the article if you think it'll be useful to someone + join the discussion about this post on Twitter!


Stay in touch!



About the author

Khalil Stemmler

Khalil Stemmler is a Developer / Designer and co-founder of Univjobs. He frequently publishes articles about Domain-Driven Design and Advanced TypeScript & Node.js best practices for large-scale applications.


View more in Web Development


You may also enjoy...

A few more related articles

Fixing Sequelize Models with Migrations
Jul 17th, 2018 / 8 min read
Sequelize is a great ORM for NodeJS applications that are built on relational backends. Inevitably, you'll need to update your mod...
Over $85 billion spent on fixing bad code [bootcamps, junior devs, JavaScript, and software design principles]
Jun 7th, 2019 / 10 min read
More and more money is being spent by companies on maintaining bad JavaScript code. Here's where I think our industry needs to cha...
TypeScript vs. JavaScript [Pros / cons]
May 11th, 2019 / 4 min read
TypeScript is arguably one of the best things to come to the JavaScript world helping developers comfortably create complex enterp...
How to Build a Related Posts Component with Gatsby.js
Apr 16th, 2019 / 7 min read
Here's how to build a "Related Posts" component for your Gatsby.js site.

Want to be notified when new content comes out?

Join 2000+ other developers learning about Domain-Driven Design and Enterprise Node.js.

I won't spam ya. 🖖 Unsubscribe anytime.

Get updates