Relations

Kysely IS NOT an ORM. Kysely DOES NOT have the concept of relations. Kysely IS a query builder. Kysely DOES build the SQL you tell it to, nothing more, nothing less.

Phew, glad we got that out the way..

All that was said above doesn't mean there's no way to nest related rows in your queries. You just have to do it with the tools SQL and the underlying dialect (e.g. PostgreSQL or MySQL) provide. In this recipe we show one way to do that when using the built-in PostgreSQL and MySQL dialects.

The json data type and functions

Both PostgreSQL and MySQL have rich JSON support through their json data types and functions. pg and mysql2, the node drivers, automatically parse returned json columns as json objects. With the combination of these two things, we can write some super efficient queries with nested relations.

Let's start with some raw postgres SQL, and then see how we can write the query using Kysely in a nice type-safe way.

In the following query, we fetch a list of people (from "person" table) and for each person, we nest the person's pets, and mother, into the returned objects:

SELECT
  person.*,

  -- Select person's pets as a json array
  (
    SELECT 
      COALESCE(JSON_AGG(pets), '[]')
    FROM
    (
      SELECT 
        pet.id, pet.name
      FROM
        pet
      WHERE 
        pet.owner_id = person.id
      ORDER BY 
        pet.name
    ) pets
  ) pets,

  -- Select person's mother as a json object
  (
    SELECT 
      TO_JSON(mother)
    FROM
    (
      SELECT 
        mother.id, mother.first_name
      FROM
        person as mother
      WHERE 
        mother.id = person.mother_id
    ) mother
  ) mother
FROM
  person

Simple right 😅. Yeah, not so much. But it does provide full control over the queries and a really good performance as long as you have indices (or indexes, we don't judge) for "pet.owner_id" and "person.mother_id".

Fortunately we can improve and simplify this a lot using Kysely. First let's define a couple of helpers:

function jsonArrayFrom<O>(
  expr: Expression<O>
): RawBuilder<Simplify<O>[]> {
  return sql`(select coalesce(json_agg(agg), '[]') from ${expr} as agg)`
}

export function jsonObjectFrom<O>(
  expr: Expression<O>
): RawBuilder<Simplify<O>> {
  return sql`(select to_json(obj) from ${expr} as obj)`
}

These helpers are included in Kysely and you can import them from the helpers module like this:

import { jsonArrayFrom, jsonObjectFrom } from 'kysely/helpers/postgres'

MySQL versions of the helpers are slightly different but you can use them the same way. You can import them like this:

import { jsonArrayFrom, jsonObjectFrom } from 'kysely/helpers/mysql'

With these helpers, our example query already becomes a little more bearable to look at:

const persons = await db
  .selectFrom('person')
  .selectAll('person')
  .select((eb) => [
    // pets
    jsonArrayFrom(
      eb.selectFrom('pet')
        .select(['pet.id', 'pet.name'])
        .whereRef('pet.owner_id', '=', 'person.id')
        .orderBy('pet.name')
    ).as('pets'),

    // mother
    jsonObjectFrom(
      eb.selectFrom('person as mother')
        .select(['mother.id', 'mother.first_name'])
        .whereRef('mother.id', '=', 'person.mother_id')
    ).as('mother')
  ])

console.log(persons[0].pets[0].name)
console.log(persons[0].mother.first_name)

That's better right? If you need to do this over and over in your codebase, you can create some helpers like these:

function withPets(eb: ExpressionBuilder<DB, 'person'>) {
  return jsonArrayFrom(
    eb.selectFrom('pet')
      .select(['pet.id', 'pet.name'])
      .whereRef('pet.owner_id', '=', 'person.id')
      .orderBy('pet.name')
  ).as('pets')
}

function withMom(eb: ExpressionBuilder<DB, 'person'>) {
  return jsonObjectFrom(
    eb.selectFrom('person as mother')
      .select(['mother.id', 'mother.first_name'])
      .whereRef('mother.id', '=', 'person.mother_id')
  ).as('mother')
}

And now you get this:

const persons = await db
  .selectFrom('person')
  .selectAll('person')
  .select((eb) => [
    withPets(eb),
    withMom(eb)
  ])

console.log(persons[0].pets[0].name)
console.log(persons[0].mother.first_name)