[y0608]

Closes #2723

Allows the new action to build the resource from request params, so the form can be prefilled:

/admin/customers/new?customer[name]=Bob

Use cases:

• Linking to a prefilled form, e.g. "New city" from a country page: /admin/cities/new?city[country_id]=1
• Dynamic dropdowns: reload the form with the selected value to filter dependent selects (demo below)

Params go through resource_params, so only the dashboard's permitted attributes are used. Requests without params behave as before. The check uses param_key, so namespaced models (Blog::Post → blog_post) work too.

Demo app

Creates a Rails app using this branch, with Country / City / Person. The city select on the new person form is filtered by the selected country, which only works because new now reads params.

#!/usr/bin/env bash
set -e
rails new prefill_demo --skip-test --skip-system-test && cd prefill_demo
bundle add administrate --git https://github.com/y0608/administrate --branch prefill-new-action-params

bin/rails g model Country name
bin/rails g model City name country:references
bin/rails g model Person name country:references city:references
echo 'class Country < ApplicationRecord
  has_many :cities
end' > app/models/country.rb
echo 'class Person < ApplicationRecord
  belongs_to :country
  belongs_to :city
end' > app/models/person.rb
echo 'bg = Country.create!(name: "Bulgaria")
de = Country.create!(name: "Germany")
bg.cities.create!(name: "Sofia")
bg.cities.create!(name: "Plovdiv")
de.cities.create!(name: "Berlin")
de.cities.create!(name: "Munich")' > db/seeds.rb
bin/rails db:migrate db:seed

bin/rails g administrate:install
sed -i 's/^    city: Field::BelongsTo,/    city: Field::BelongsTo.with_options(scope: ->(field) { field.resource.country ? field.resource.country.cities : City.none }),/' app/dashboards/person_dashboard.rb
sed -i '$i\  def display_resource(r) = r.name' app/dashboards/country_dashboard.rb app/dashboards/city_dashboard.rb

# Reload the new form with the selected country so the city select gets filtered
bin/rails g administrate:views:form person
echo '<script>
  window.addEventListener("load", () => {
    $("#person_country_id").on("change", (e) => {
      window.location = "<%= new_admin_person_path %>?person[name]=" + encodeURIComponent($("#person_name").val()) + "&person[country_id]=" + e.target.value
    })
  })
</script>' >> app/views/admin/people/_form.html.erb

echo "Open http://localhost:3000/admin/people/new and change the country"
bin/rails s

[y0608]

If this approach looks good, I'm happy to follow up with the same behaviour for edit.