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Custom Migration Helpers ​

To build steps that the helpers do not cover, such as creating enum types, write a function that returns a step. Quote names with quoteIdentifier and string values with quoteLiteral:

ts
import type { MigrationStep } from "kern-migrations"
import { quoteIdentifier, quoteLiteral } from "kern-migrations"

export function createEnum(name: string, values: readonly string[]): MigrationStep {
  return {
    up: `CREATE TYPE ${quoteIdentifier(name)} AS ENUM (${values.map(quoteLiteral).join(", ")})`,
    down: `DROP TYPE ${quoteIdentifier(name)}`,
  }
}

Use it in migrations as a built-in helper:

ts
import { defineMigration } from "kern-migrations"

import { createEnum } from "../migration-helpers.ts"

export default defineMigration(createEnum("form_status", ["draft", "published"]))

Do not change the SQL that a helper builds once a migration uses it: a new database, such as one for a new tenant, runs the migration with the helper again and must get the same result. To change what a helper does, add a new helper or option.

Undo a step ​

To build a helper that undoes what another one does, pass its step to invertStep, which swaps up and down, also those of outsideTransaction:

ts
import { invertStep } from "kern-migrations"

export function dropEnum(name: string, values: readonly string[]): MigrationStep {
  return invertStep(createEnum(name, values))
}

Combine steps ​

To combine several steps into one, such as adding columns shared by many tables, pass them to combineSteps: its up runs theirs in order, and its down runs theirs in reverse order.

ts
import { addColumn, combineSteps } from "kern-migrations"

export function addTimestamps(table: string): MigrationStep {
  return combineSteps(
    addColumn(table, "created_at", "timestamptz NOT NULL DEFAULT now()"),
    addColumn(table, "updated_at", "timestamptz NOT NULL DEFAULT now()"),
  )
}