Support nested forms for has_many relationships

Closes #192

Feature:

When I create or edit a model that has_many nested models,
I want to view and edit the attributes of the nested models
so I can set up all the relationships with a single form.

Implementation:

Introduce a new field type, `NestedHasMany`.

I considered building the feature into the existing `HasMany` field,
but this would get in the way of `HasMany` relationships
that aren't directly nested, such as in many-to-many relationships.

The `NestedHasMany` field builds off of the [Cocoon] gem.
It renders fields from the nested model,
based on the fields defined in the nested model's dashboard class.

Cocoon provides helpers and javascript
to easily add and remove nested form fields from the page.

[Cocoon]: https://github.com/nathanvda/cocoon

administrate.gemspec

@@ -24,6 +24,7 @@ Gem::Specification.new do |s|
   s.add_dependency "rails", "~> 4.2"
   s.add_dependency "sass-rails", "~> 5.0"
   s.add_dependency "selectize-rails", "~> 0.6"
+  s.add_dependency "cocoon", "~> 1.2"
 
   s.description = <<-DESCRIPTION
 Administrate is heavily inspired by projects like Rails Admin and ActiveAdmin,

app/assets/javascripts/administrate/application.js

@@ -1,6 +1,9 @@
 //= require jquery
 //= require jquery_ujs
-//= require selectize
-//= require moment
+
+//= require cocoon
 //= require datetime_picker
+//= require moment
+//= require selectize
+
 //= require_tree .

app/views/fields/nested_has_many/_fields.html.erb

@@ -0,0 +1,12 @@
+<div class="nested-fields" style="width:100%">
+
+  <%# TODO filter out the parent relationship, without this crude array slicing %>
+  <% field.associated_form.attributes[1..-1].each do |attribute| -%>
+    <div class="field">
+      <%= render_field attribute, f: f %>
+    </div>
+  <% end -%>
+
+  <%# TODO I18n %>
+  <%= link_to_remove_association "Remove #{field.associated_class_name.titleize}", f %>
+</div>

app/views/fields/nested_has_many/_form.html.erb

@@ -0,0 +1,24 @@
+<div class="nested-fields" style="width: 40%; margin-left: 20%;">
+
+  <%= f.fields_for field.association_name do |nested_form| %>
+    <%= render(
+      partial: "fields/nested_has_many/fields",
+      locals: {
+        f: nested_form,
+        field: field,
+      },
+    ) %>
+  <% end %>
+
+  <div class="links">
+    <%= link_to_add_association(
+      # TODO I18n
+      "Add #{field.associated_class_name.titleize}",
+      f,
+      field.association_name,
+      partial: "fields/nested_has_many/fields",
+      render_options: { locals: { field: field } },
+    ) %>
+  </div>
+
+</div>

app/views/fields/nested_has_many/_index.html.erb

@@ -0,0 +1,19 @@
+<%#
+# HasMany Index Partial
+
+This partial renders a has_many relationship,
+to be displayed on a resource's index page.
+
+By default, the relationship is rendered
+as a count of how many objects are associated through the relationship.
+
+## Local variables:
+
+- `field`:
+  An instance of [Administrate::Field::HasMany][1].
+  A wrapper around the has_many relationship pulled from the database.
+
+[1]: http://www.rubydoc.info/gems/administrate/Administrate/Field/HasMany
+%>
+
+<%= pluralize(field.data.size, field.attribute.to_s.humanize.downcase) %>

app/views/fields/nested_has_many/_show.html.erb

@@ -0,0 +1,40 @@
+<%#
+# HasMany Show Partial
+
+This partial renders a has_many relationship,
+to be displayed on a resource's show page.
+
+By default, the relationship is rendered
+as a table of the first few associated resources.
+The columns of the table are taken
+from the associated resource class's dashboard.
+
+## Local variables:
+
+- `field`:
+  An instance of [Administrate::Field::HasMany][1].
+  Contains methods to help display a table of associated resources.
+
+[1]: http://www.rubydoc.info/gems/administrate/Administrate/Field/HasMany
+%>
+
+<% if field.resources.any? %>
+  <%= render(
+    "collection",
+    collection_presenter: field.associated_collection,
+    resources: field.resources
+  ) %>
+
+  <% if field.more_than_limit? %>
+    <span>
+      <%= t(
+        'administrate.fields.has_many.more',
+        count: field.limit,
+        total_count: field.data.count,
+      ) %>
+    </span>
+  <% end %>
+
+<% else %>
+  <%= t("administrate.fields.has_many.none") %>
+<% end %>

lib/administrate/base_dashboard.rb

@@ -5,6 +5,7 @@
 require "administrate/field/has_many"
 require "administrate/field/has_one"
 require "administrate/field/image"
+require "administrate/field/nested_has_many"
 require "administrate/field/number"
 require "administrate/field/polymorphic"
 require "administrate/field/select"

lib/administrate/engine.rb

@@ -1,3 +1,4 @@
+require "cocoon"
 require "datetime_picker_rails"
 require "jquery-rails"
 require "kaminari"

lib/administrate/field/nested_has_many.rb

@@ -0,0 +1,46 @@
+require_relative "has_many"
+require "administrate/page/form"
+
+module Administrate
+  module Field
+    class NestedHasMany < Administrate::Field::HasMany
+      DEFAULT_ATTRIBUTES = [:id, :_destroy].freeze
+
+      def to_s
+        data
+      end
+
+      protected
+
+      def self.dashboard_for_resource(resource)
+        "#{resource.to_s.classify}Dashboard".constantize
+      end
+
+      def self.associated_attributes(associated_resource)
+        DEFAULT_ATTRIBUTES +
+          dashboard_for_resource(associated_resource).new.permitted_attributes
+      end
+
+      public
+
+      def self.permitted_attribute(associated_resource)
+        {
+          "#{associated_resource}_attributes".to_sym =>
+          associated_attributes(associated_resource)
+        }
+      end
+
+      def associated_class_name
+        options.fetch(:class_name, attribute.to_s.singularize.camelcase)
+      end
+
+      def association_name
+        associated_class_name.underscore.pluralize
+      end
+
+      def associated_form
+        Administrate::Page::Form.new(associated_dashboard, association_name)
+      end
+    end
+  end
+end