First draft of API to add items to a collection (#37222)

app/controllers/api/v1_alpha/collection_items_controller.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+class Api::V1Alpha::CollectionItemsController < Api::BaseController
+  include Authorization
+
+  before_action :check_feature_enabled
+
+  before_action -> { doorkeeper_authorize! :write, :'write:collections' }
+
+  before_action :require_user!
+
+  before_action :set_collection
+  before_action :set_account, only: [:create]
+
+  after_action :verify_authorized
+
+  def create
+    authorize @collection, :update?
+    authorize @account, :feature?
+
+    @item = AddAccountToCollectionService.new.call(@collection, @account)
+
+    render json: @item, serializer: REST::CollectionItemSerializer
+  end
+
+  private
+
+  def set_collection
+    @collection = Collection.find(params[:collection_id])
+  end
+
+  def set_account
+    return render(json: { error: '`account_id` parameter is missing' }, status: 422) if params[:account_id].blank?
+
+    @account = Account.find(params[:account_id])
+  end
+
+  def check_feature_enabled
+    raise ActionController::RoutingError unless Mastodon::Feature.collections_enabled?
+  end
+end