Custom Scalars

Custom scalars extend the built-in primitive types with domain-specific behavior and coercion.

Defining a scalar

app/types/custom_scalars.rb

Zodra.scalar :money, base: :decimal do |value|
  BigDecimal(value.to_s).round(2)
rescue ArgumentError
  :coercion_error
end

name — the scalar name (used as a field type method)
base: — the underlying primitive type (:string, :integer, :decimal, :datetime, etc.)
&block — coercion function that transforms raw input values

Return :coercion_error from the block to signal that the value cannot be coerced. Zodra will report a validation error for that field.

Once registered, the scalar becomes a field type method:

app/types/product.rb

Zodra.type :product do
  uuid :id
  string :name
  money :price          # required
  money? :discount      # optional
end

Custom scalars generate their base type in TypeScript. A money scalar with base: :decimal generates z.number():

export const ProductSchema = z.object({
  id: z.uuid(),
  name: z.string(),
  price: z.number(),
  discount: z.number().optional(),
});

Zodra.scalar :json, base: :string do |value|
  value.is_a?(String) ? JSON.parse(value) : value
rescue JSON::ParserError
  :coercion_error
end

Zodra.scalar :slug, base: :string do |value|
  value.to_s.downcase.gsub(/[^a-z0-9\-]/, "-").gsub(/-+/, "-")
end