heartcombo/simple_form · error · ArgumentError

Association cannot be used in forms not associated with an…

Error message

Association cannot be used in forms not associated with an object

What it means

f.association needs the form's @object to read the ActiveRecord reflection, infer the attribute name (e.g. category_id) and fetch the collection. If the builder has no object — as with simple_form_for :search — and no block is given, it raises ArgumentError (lib/simple_form/form_builder.rb:213). With a block it delegates to simple_fields_for instead and never raises.

Solutions

For object-less forms, replace association with an explicit collection input: f.input :category_id, collection: Category.order(:name)
Back the form with a real object — a persisted or unsaved record, form object or presenter — so the reflection can be read
If you want nested fields, pass a block: f.association :category do |c| ... end, which delegates to simple_fields_for and bypasses the check

Example fix

# before
<%= simple_form_for :search, method: :get do |f| %>
  <%= f.association :category %>  <!-- ArgumentError: not associated with an object -->
<% end %>

# after
<%= simple_form_for :search, method: :get do |f| %>
  <%= f.input :category_id, collection: Category.order(:name), label_method: :name, value_method: :id %>
<% end %>

Defensive patterns

Strategy: validation

Validate before calling

if f.object.present?
  f.association :category
else
  f.input :category_id, collection: Category.order(:name)
end

Prevention

Reserve f.association for forms bound to a record instance
For search/filter forms, use f.input with an explicit :collection
In shared partials, branch on f.object.present? so both object-backed and object-less forms render
Set controller instance variables explicitly so simple_form_for @record never receives nil unexpectedly

When it happens

Trigger: simple_form_for :search, method: :get do |f| f.association :category end (symbol form, object is nil); simple_form_for(@search) where @search is nil because the controller never assigned it; any builder whose object is nil calling association without a block.

Common situations: Search/filter forms that only need a dropdown of records; shared form partials reused for both model-backed and object-less forms; controller refactor that stops setting the instance variable the view builds its form from.

Related errors

AI-assisted analysis of heartcombo/simple_form@18f38aad0b (2026-08-21). Data as JSON: /api/errors/7c4c0fd55acfc4ef. Report an issue: GitHub.

Appendix: source

Thrown at lib/simple_form/form_builder.rb:213

    # When a block is given, association simple behaves as a proxy to
    # simple_fields_for:
    #
    #   f.association :company do |c|
    #     c.input :name
    #     c.input :type
    #   end
    #
    # From the options above, only :collection can also be supplied.
    #
    # Please note that the association helper is currently only tested with Active Record. Depending on the ORM you are using your mileage may vary.
    #
    def association(association, options = {}, &block)
      options = options.dup

      return simple_fields_for(*[association,
        options.delete(:collection), options].compact, &block) if block_given?

      raise ArgumentError, "Association cannot be used in forms not associated with an object" unless @object

      reflection = find_association_reflection(association)
      raise "Association #{association.inspect} not found" unless reflection

      options[:as] ||= :select
      options[:collection] ||= fetch_association_collection(reflection, options)

      attribute = build_association_attribute(reflection, association, options)

      input(attribute, options.merge(reflection: reflection))
    end

    # Creates a button:
    #
    #   form_for @user do |f|
    #     f.button :submit
    #   end
    #

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