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Models in your application are rarely isolated. Esix ships with three ActiveRecord-style helpers that make it easy to traverse relationships between collections: hasMany, hasOne, and belongsTo. Each helper follows the same convention: by default, the foreign key is the camelCased class name with Id appended (e.g. Author → authorId), and the local/owner key is id. Both keys can be overridden when your schema differs.

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hasMany

Use hasMany when a parent record can own many related records. It returns a QueryBuilder, so you can chain additional constraints before fetching: 
class Author extends BaseModel {
  public name = ''

  posts() {
    return this.hasMany(Post)
  }
}

class Post extends BaseModel {
  public title = ''
  public authorId = ''
  public publishedAt: number | null = null
}

const author = await Author.find('5f5a474b32fa462a5724ff7d')

// All posts by this author.
const allPosts = await author.posts().get()

// Only the published ones, latest first.
const recentPosts = await author
  .posts()
  .where('publishedAt', '!=', null)
  .orderBy('publishedAt', 'desc')
  .limit(5)
  .get()
author 
{
  id: '5f5a474b32fa462a5724ff7d',
  name: 'Ada Lovelace'
}
allPosts
idtitleauthorIdpublishedAt
6011a52b9f1b2c4d8e7f3a21Notes on the Analytical Engine5f5a474b32fa462a5724ff7d1736380800000
60119e8a9f1b2c4d8e7f3a14Bernoulli Numbers by Algorithm5f5a474b32fa462a5724ff7d1735862400000
601198119f1b2c4d8e7f3a09Loose Sketches and Stray Ideas5f5a474b32fa462a5724ff7dnull
recentPosts
idtitleauthorIdpublishedAt
6011a52b9f1b2c4d8e7f3a21Notes on the Analytical Engine5f5a474b32fa462a5724ff7d1736380800000
60119e8a9f1b2c4d8e7f3a14Bernoulli Numbers by Algorithm5f5a474b32fa462a5724ff7d1735862400000
You can pass a custom foreign key, local key, or both: 
this.hasMany(Post, 'writtenBy') // foreignKey only
this.hasMany(Post, 'writtenBy', 'externalId') // foreignKey + localKey

​
hasOne

hasOne mirrors hasMany but returns a single record (or null). It’s the right choice for true one-to-one relationships such as a user and their profile: 
class User extends BaseModel {
  public name = ''

  profile() {
    return this.hasOne(Profile)
  }
}

class Profile extends BaseModel {
  public bio = ''
  public userId = ''
}

const user = await User.find('user-123')
const profile = await user.profile() // Profile | null
user 
{
  id: 'user-123',
  name: 'Grace Hopper'
}
profile 
{
  id: '6011a52b9f1b2c4d8e7f3a09',
  bio: 'Compiler pioneer. Loves COBOL and nanoseconds.',
  userId: 'user-123'
}
Like hasMany, you can override the foreign and local keys: 
this.hasOne(Profile, 'ownerId', 'externalId')

​
belongsTo

belongsTo is the inverse of hasOne / hasMany: it looks up the parent record from a foreign key stored on the current model. 
class Post extends BaseModel {
  public title = ''
  public authorId = ''

  author() {
    return this.belongsTo(Author)
  }
}

const post = await Post.find('post-1')
const author = await post.author() // Author | null
post 
{
  id: 'post-1',
  title: 'Notes on the Analytical Engine',
  authorId: '5f5a474b32fa462a5724ff7d'
}
author 
{
  id: '5f5a474b32fa462a5724ff7d',
  name: 'Ada Lovelace'
}
By default belongsTo uses the parent’s id as the owner key and looks up the foreign key derived from the parent class name. Override either if your schema uses different fields: 
// Look up Author by `slug` instead of `id`.
this.belongsTo(Author, 'authorSlug', 'slug')
If the foreign key is null or undefined on the current model, belongsTo returns null without hitting the database.