Use allow_destroy. Remove condtion from form.haml.

app/controllers/projects/triggers_controller.rb

@@ -16,6 +16,7 @@ class Projects::TriggersController < Projects::ApplicationController
     if @trigger.valid?
       flash[:notice] = 'Trigger was created successfully.'
     else
+      puts "@trigger.errors: #{@trigger.errors.inspect}"
       flash[:alert] = 'You could not create a new trigger.'
     end
 
@@ -33,6 +34,7 @@ class Projects::TriggersController < Projects::ApplicationController
   end
 
   def edit
+    @trigger.build_trigger_schedule unless @trigger.trigger_schedule.present?
   end
 
   def update
@@ -69,15 +71,15 @@ class Projects::TriggersController < Projects::ApplicationController
 
   def create_params
     params.require(:trigger).permit(
-      :description, :ref, :trigger_schedule_on,
-      trigger_schedule_attributes: [ :cron, :cron_timezone ]
+      :description, :ref,
+      trigger_schedule_attributes: [ :cron, :cron_timezone, :_destroy ]
     )
   end
 
   def update_params
     params.require(:trigger).permit(
-      :description, :ref, :trigger_schedule_on,
-      trigger_schedule_attributes: [ :cron, :cron_timezone ]
+      :description, :ref,
+      trigger_schedule_attributes: [ :cron, :cron_timezone, :_destroy ]
     )
   end
 end

app/models/ci/trigger.rb

@@ -8,27 +8,16 @@ module Ci
     belongs_to :owner, class_name: "User"
 
     has_many :trigger_requests, dependent: :destroy
-    has_one :trigger_schedule, dependent: :destroy
+    has_one :trigger_schedule, dependent: :destroy, inverse_of: :trigger
 
     validates :token, presence: true, uniqueness: true
 
     before_validation :set_default_values
 
-    accepts_nested_attributes_for :trigger_schedule
-
-    attr_accessor :trigger_schedule_on
+    accepts_nested_attributes_for :trigger_schedule, allow_destroy: true
 
     def set_default_values
       self.token = SecureRandom.hex(15) if self.token.blank?
-
-      if trigger_schedule_on.present?
-        if trigger_schedule_on.to_i == 1
-          self.trigger_schedule.project = project
-          self.trigger_schedule.trigger = self
-        else
-          self.trigger_schedule = nil
-        end
-      end
     end
 
     def last_trigger_request

app/models/ci/trigger_schedule.rb

@@ -6,11 +6,11 @@ module Ci
     acts_as_paranoid
 
     belongs_to :project
-    belongs_to :trigger
+    belongs_to :trigger, inverse_of: :trigger_schedule
 
     delegate :ref, to: :trigger, allow_nil: true
 
-    validates :trigger, presence: { unless: :importing? }
+    validates_presence_of :trigger
     validates :cron, cron: true, presence: { unless: :importing? }
     validates :cron_timezone, cron_timezone: true, presence: { unless: :importing? }
     validates :ref, presence: { unless: :importing? }
@@ -19,6 +19,7 @@ module Ci
 
     def set_next_run_at
       self.next_run_at = Gitlab::Ci::CronParser.new(cron, cron_timezone).next_time_from(Time.now)
+      self.project = trigger.project
     end
 
     def schedule_next_run!

app/views/projects/triggers/_form.html.haml

@@ -18,13 +18,10 @@
     .form-group
       = f.label :ref, "Ref (For scheduled trigger)", class: "label-light"
       = f.text_field :ref, class: "form-control", title: 'Trigger Schedule Ref is required.', placeholder: "master"
-  - if action_name == 'edit'
-    = f.hidden_field :trigger_schedule_on, :value => @trigger.trigger_schedule.present? ? 1 : 0
-  - else
     .form-group
       .checkbox
-        = f.label :trigger_schedule_on do
-          = f.check_box :trigger_schedule_on
+        = schedule_fields.label :_destroy do
+          = schedule_fields.check_box :_destroy, { checked: (@trigger.trigger_schedule.id.present?) }, 0, 1
           %strong Register as scheduled trigger
         .help-block
           If checked, this trigger will be executed periodically according to `cron`, `cron_timezone` and `ref`