Reform has two completely separated modes for form setup. One when rendering the form and one when populating the form in validate.

Prepopulating is helpful when you want to fill out fields (aka. defaults) or add nested forms before rendering.

Populating is invoked in validate and will add nested forms depending on the incoming hash.

This page explains prepopulation used to prepare the form for rendering.


You can use the :prepopulator option on every property or collection.

class AlbumForm < Reform::Form
  property :artist, prepopulator: ->(options) { self.artist = } do
    property :name

The option value can be a lambda or an instance method name.

In the block/method, you have access to the form API and can invoke any kind of logic to prepopulate your form. Note you need to assign models for nested form using their writers.


Prepopulation must be invoked manually.

form =
form.artist #=> nil


form.artist #=> <nested ArtistForm @model=<Artist ..>>

This explicit call must happen before the form gets rendered. For instance, in Trailblazer, this happens in the controller action.

Prepopulate is not Populate

:populator and :populate_if_empty will be run automatically in validate. Do not call prepopulate! before validate if you use the populator options. This will usually result in “more” nested forms being added as you wanted (unless you know what you’re doing).

Prepopulators are a concept designed to prepare a form for rendering, whereas populators are meant to set up the form in validate when the input hash is deserialized.

This is explained in the Nested Forms chapter of the Trailblazer book. Please read it first if you have trouble understanding this, and then open an issue.


Options may be passed. They will be available in the :prepopulator block.

class AlbumForm < Reform::Form
  property :title, prepopulator: ->(options) { self.title = options[:def_title] }

You can then pass arbitrary arguments to prepopulate!.

form.title #=> nil

form.prepopulate!(def_title: "Roxanne")

form.title #=> "Roxanne"

The arguments passed to the prepopulate! call will be passed straight to the block/method.

This call will be applied to the entire nested form graph recursively after the currently traversed form’s prepopulators were run.


The blocks are run in form instance context, meaning you have access to all possible data you might need. With a symbol, the same-named method will be called on the form instance, too.

Note that you have to assign the pre-populated values to the form by using setters. In turn, the form will automatically create nested forms for you.

This is especially cool when populating collections.

property :songs,
  prepopulator: ->(*) { self.songs << if songs.size < 3 } do

This will always add an empty song form to the nested songs collection until three songs are attached. You can use the Twin::Collection API when adding, changing or deleting items from a collection.

Note that when calling #prepopulate!, your :prepopulate code for all existing forms in the graph will be executed . It is up to you to add checks if you need that.


You don’t have to use the :prepopulator option. Instead, you can simply override #prepopulate! itself.

class AlbumForm < Reform::Form
  def prepopulate!(options)
    self.title = "Roxanne"
    self.artist = "The Police")


There’s different alternatives for setting a default value for a formerly empty field.

  1. Use :prepopulator as described here. Don’t forget to call prepopulate! before rendering the form.
  2. Override the reader of the property. This is not recommended as you might screw things up. Remember that the property reader is called for presentation (in the form builder) and for validation in #validate.

    property :title
    def title
      super or "Unnamed"