_form.html.erb

# views/fields/belongs_to_scoped/_form.html.erb

<div class="field-unit__label">
  <%= f.label field.permitted_attribute %>
</div>
<div class="field-unit__field">
  <%= f.select(field.permitted_attribute) do %>
    <%= options_for_select(field.associated_resource_options(f.object.send("#{field.name}_candidates")), field.selected_option) %>
  <% end %>
</div>

belongs_to_scoped.rb

class BelongsToScoped < Administrate::Field::BelongsTo
  def associated_resource_options(candidate_resources)
    [nil] + candidate_resources.map do |resource|
      [display_candidate_resource(resource), resource.send(primary_key)]
    end
  end
  def to_partial_path
    case page
    when :form; "/fields/belongs_to_scoped/form"
    else "/fields/belongs_to/#{page}"
    end
  end
end

user.rb

# example

class User < ApplicationRecord
  belongs_to :group
  belongs_to :user
  
  # this method is called in form/fields/belongs_to_scoped/_form
  def user_candidates
    group ? User.where(locale: group.locale) : User.all
  end
end

user_group_dashboard.rb

# example

#...
  ATTRIBUTE_TYPES = {
    group: Field::BelongsTo,
    user: BelongsToScoped
  }
#...