Draft session dates to prevent save on "back"

app/controllers/session_dates_controller.rb

@@ -2,41 +2,33 @@
 
 class SessionDatesController < ApplicationController
   before_action :set_session
+  before_action :set_draft_session_dates
 
   def show
-    @session.session_dates.build if @session.session_dates.empty?
-  end
-
-  def update
-    @session.assign_attributes(remove_invalid_dates(session_params))
-    @session.set_notification_dates
+    initialize_draft_from_session
 
-    render :show, status: :unprocessable_content and return if @session.invalid?
-
-    @session.save!
-
-    # If deleting dates, they don't disappear from `session.dates` until
-    # the model has been saved due to how `accepts_nested_attributes_for`
-    # works.
-    if any_destroyed?
-      @session.session_dates.reload
-      @session.set_notification_dates
-      @session.save!
+    if params[:add_another] ||
+         @draft_session_dates.session_dates_attributes.empty?
+      add_blank_session_date
     end
+  end
 
-    StatusUpdaterJob.perform_later(session: @session)
+  def update
+    @draft_session_dates.session_dates_attributes =
+      fetch_draft_session_dates_from_form
 
-    if params.include?(:add_another)
-      @session.session_dates.build
+    if params[:delete_date]
+      delete_session_date(form_index_to_attr_index(params[:delete_date]))
+      return render_with_error if @draft_session_dates.errors.any?
+      render :show
+    elsif params[:add_another]
+      add_blank_session_date
       render :show
     else
-      redirect_to(
-        if any_destroyed?
-          session_dates_path(@session)
-        else
-          edit_session_path(@session)
-        end
-      )
+      unless @draft_session_dates.save(context: :continue)
+        return render_with_error
+      end
+      apply_changes_and_redirect
     end
   end
 
@@ -46,41 +38,121 @@ def set_session
     @session = policy_scope(Session).find_by!(slug: params[:session_slug])
   end
 
-  def session_params
-    params.expect(
-      session: {
-        session_dates_attributes: [%i[id value _destroy]]
-      }
+  def set_draft_session_dates
+    @draft_session_dates =
+      DraftSessionDates.new(
+        request_session: session,
+        current_user: current_user,
+        session: @session,
+        wizard_step: :dates
+      )
+  end
+
+  def draft_session_dates_params
+    params
+      .fetch(:draft_session_dates, {})
+      .permit(
+        session_dates_attributes: [
+          :id,
+          :value,
+          :_destroy,
+          "value(1i)",
+          "value(2i)",
+          "value(3i)"
+        ]
+      )
+      .merge(wizard_step: :dates)
+  end
+
+  def initialize_draft_from_session
+    attributes = {}
+
+    @session
+      .session_dates
+      .order(:value)
+      .each_with_index do |session_date, index|
+        attributes[index] = {
+          "id" => session_date.id.to_s,
+          "value" => session_date.value
+        }
+      end
+
+    @draft_session_dates.session_dates_attributes = attributes
+    @draft_session_dates.save!(context: :initialize)
+  end
+
+  def add_blank_session_date
+    current_attrs = @draft_session_dates.session_dates_attributes
+    next_index = (current_attrs.keys.map(&:to_i).max || -1) + 1
+    current_attrs[next_index] = { "value" => nil }
+    @draft_session_dates.session_dates_attributes = current_attrs
+  end
+
+  def render_with_error
+    render :show, status: :unprocessable_content
+  end
+
+  def apply_changes_and_redirect
+    @draft_session_dates.write_to!(@session)
+    session.delete(@draft_session_dates.request_session_key)
+    StatusUpdaterJob.perform_later(session: @session)
+    redirect_to edit_session_path(@session)
+  rescue ActiveRecord::RecordInvalid => e
+    @draft_session_dates.errors.add(
+      :base,
+      "Failed to save session dates: #{e.message}"
     )
+    render_with_error
   end
 
-  def any_destroyed?
-    session_params[:session_dates_attributes].values.any? do
-      _1[:_destroy].present?
+  def form_index_to_attr_index(form_index)
+    current_attrs = @draft_session_dates.session_dates_attributes
+    corresponding_form_index = -1
+    current_attrs.each do |real_index, attrs|
+      corresponding_form_index += 1 unless attrs["_destroy"] == "true"
+
+      return real_index if corresponding_form_index.to_s == form_index
     end
+
+    (
+      form_index.to_i - corresponding_form_index.to_i + current_attrs.length
+    ).to_s
   end
 
-  def remove_invalid_dates(obj, key: "session_dates_attributes")
-    return obj if obj[key].blank?
-
-    obj[key] = obj[key].transform_values do |value|
-      if value.key?("value(1i)") && value.key?("value(2i)") &&
-           value.key?("value(3i)")
-        begin
-          Date.new(
-            value["value(1i)"].to_i,
-            value["value(2i)"].to_i,
-            value["value(3i)"].to_i
-          )
-          value
-        rescue StandardError
-          {}
-        end
-      else
-        value
-      end
+  def fetch_draft_session_dates_from_form
+    current_attrs = @draft_session_dates.session_dates_attributes
+
+    form_params = draft_session_dates_params[:session_dates_attributes] || {}
+
+    merged_attrs = {}
+
+    current_attrs.each do |index, attrs|
+      merged_attrs[index] = attrs if attrs["_destroy"] == "true"
     end
 
-    obj
+    form_params.each do |form_index, form_attrs|
+      attr_index = form_index_to_attr_index(form_index)
+      merged_attrs[attr_index] = form_attrs
+    end
+
+    merged_attrs
+  end
+
+  def delete_session_date(index)
+    current_attrs = @draft_session_dates.session_dates_attributes
+
+    if current_attrs[index]
+      if @draft_session_dates.non_destroyed_session_dates_count <= 1
+        @draft_session_dates.errors.add(
+          :base,
+          "You cannot delete the last session date. A session must have at least one date."
+        )
+        return
+      end
+
+      current_attrs[index]["_destroy"] = "true"
+      @draft_session_dates.session_dates_attributes = current_attrs
+      @draft_session_dates.save!(context: :update)
+    end
   end
 end