heartcombo/simple_form · error · ArgumentError

You need to give :to as option to map_type

Error message

You need to give :to as option to map_type

What it means

map_type (from SimpleForm::MapType, extended by SimpleForm::FormBuilder — see form_builder.rb:18,21-35) registers mappings from attribute types to input classes. The trailing options hash must contain :to, the target input; without it there is nothing to map to, so ArgumentError is raised immediately at configuration time (lib/simple_form/map_type.rb:13).

Source

Thrown at lib/simple_form/map_type.rb:13

# frozen_string_literal: true
require 'active_support/core_ext/class/attribute'

module SimpleForm
  module MapType
    def self.extended(base)
      base.class_attribute :mappings
      base.mappings = {}
    end

    def map_type(*types)
      map_to = types.extract_options![:to]
      raise ArgumentError, "You need to give :to as option to map_type" unless map_to
      self.mappings = mappings.merge types.each_with_object({}) { |t, m| m[t] = map_to }
    end
  end
end

View on GitHub (pinned to 18f38aad0b)

Solutions

1. Pass the target as the :to option: SimpleForm::FormBuilder.map_type :money, to: :string
2. If you want a custom input class, point :to at it: map_type :money, to: MoneyInput (the class must be loaded)
3. Re-check the syntax against the built-in mappings in lib/simple_form/form_builder.rb:21-35

Example fix

# before
SimpleForm::FormBuilder.map_type :money
# ArgumentError: You need to give :to as option to map_type

# after
SimpleForm::FormBuilder.map_type :money, to: :string
# or target a custom input class
SimpleForm::FormBuilder.map_type :money, to: MoneyInput

# keep all custom mappings in one hash so :to can never be forgotten
custom_mappings = { money: :string, ssn: :password }
custom_mappings.each do |type, target|
  raise ArgumentError, "map_type :#{type} is missing :to" if target.nil?
  SimpleForm::FormBuilder.map_type type, to: target
end

When it happens

Trigger: SimpleForm::FormBuilder.map_type :money with no trailing hash; passing the target positionally (map_type :money, MoneyInput) instead of as to:; passing to: nil; a typo like into: or input: instead of to:.

Common situations: Copy-pasting custom-input docs and dropping the trailing options hash; adding a type mapping during an upgrade and forgetting the target; assuming map_type infers a default target input.

Related errors

• A block is required as argument to wrapper
• Couldn't find wrapper with name #{name}
• Association cannot be used in forms not associated with an o
• :has_one associations are not supported by f.association
• input should be implemented by classes inheriting from Colle

AI-assisted analysis of heartcombo/simple_form@18f38aad0b (2026-08-21). Data as JSON: /api/errors/3fa422ca9fa12998. Report an issue: GitHub.