Nithin Bekal

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Recurring Events in Rails

21 Aug 2016

Modeling recurring events in a calendar is an interesting problem to solve. There are many different scenarios in which you might need to model recurring events, but in this article I will walk through a simple example of weekly recurring events in a Rails app.

Although this post covers most of the code needed for the example, I will skip over some of the details (such as views), and you will have to fill in the blanks in such places.

Much of the code here is taken from a real project I’m working on, and I’ve tried to simplify the code as much as possible. However, some of the tradeoffs that I needed in my project aren’t really needed in this example. I have pointed out some cases where this is true.

Scheduling weekly events

Let’s take an example where we will need to model weekly recurring events:

Our users want to be able to save weekly events, with a day of the week and a time. The system will perform an action every week on that day and time. For example, a user could set up a reminder (“remind me to send status report at 5pm every Friday”), and the system should send an email at that time every week.

Modeling recurring events

Before we start writing the code for scheduling events, let’s briefly think about how we will implement this.

We will start with a RecurringEvent model, which will contain the day and time at which the event must be performed. We could also call this model something like Reminder, but since we could have other things in future that could be recurring, I prefer having a RecurringEvent model and associate that with other models. To keep things simple in this post, we will just have a reminder string field in the RecurringEvent model.

The user will create a recurring event using a form that contains the following fields:

  • reminder (eg. “Send weekly status report”)
  • time - we will just save a string containing the time, (eg. “5:00 pm”)
  • day - day of the week, as an integer (0 for Sunday, 1 for Monday, etc)

We will use an Event model to save the actual time at which a RecurringEvent occurs. We will save the next instance of the recurring event in the Event model.

We will also use ActiveJob to schedule the event at the correct time using the RunEventJob class. This ActiveJob class will execute the action that should happen at the time (sending the reminder email, in this example), and also create the next instance of the event.

Creating a new RecurringEvent

Let’s start off by creating the RecurringEvent model:

# rails generate model RecurringEvent \
# 	reminder:string time:string day:integer user:belongs_to

# app/models/recurring_event.rb
class RecurringEvent < ApplicationRecord
  belongs_to :user
  has_many :events, dependent: :destroy

  TIME_12H_FORMAT = /\A(1[0-2]|0?[1-9]):[0-5][0-9]\s?(am|pm)\z/i

  validates :time, presence: true,
    format: { with: TIME_12H_FORMAT, message: 'invalid time - use format 10:00 am' }
end

We will start with a new page for creating a new recurring event by implementing RecurringEventsController#new action.

class RecurringEventsController < ApplicationController
  before_action :authenticate_user!

  def new
    @recurring_event = RecurringEvent.new
  end
end

The form (app/views/recurring_events/new.html.haml) for adding a new recurring event looks like this:

<%= form_for(recurring_event) do |f| %>
  <div class="field">
    <%= f.label :day %>
    <%= f.select :day, Date::DAYNAMES.zip(0..6) %>
  </div>

  <div class="field">
    <%= f.label :time %>
    <%= f.text_field :time %>
  </div>

  <div class="field">
    <%= f.label :reminder %>
    <%= f.text_field :reminder %>
  </div>

  <div class="actions">
    <%= f.submit 'Save', class: 'btn btn-primary' %>
  </div>
<% end %>

The next thing to do is to implement the RecurringEventsController#create action. This is where things get a little interesting.

# app/controllers/recurring_events_controller.rb
class RecurringEvent < ApplicationRecord
  # ...

  def create
    @recurring_event =
      RecurringEvents::Create.new(current_user, recurring_event_params).call

    if @recurring_event.persisted?
      redirect_to @recurring_event, notice: 'Event was successfully created.'
    else
      render :new
    end
  end

  private
  def recurring_event_params
    params.require(:recurring_event).permit(:day, :time, :reminder)
  end
end

I like to put this kind of business logic code in app/services. RecurringEvents::Create must do three things:

  • save the recurring event
  • calculate the date of next instance of the event and save it
  • schedule a background job to run at that time

We will come back to the RecurringEvents::Create class, but first we need an Event model where we store the next occurence of a recurring event.

# rails generate model Event run_at:datetime recurring_event:belongs_to

# app/models/event.rb
class Event < ApplicationRecord
  belongs_to :recurring_event
  # also add the corresponding has_many in RecurringEvent

  scope :future, -> { where('created_at > ?', DateTime.now) }
end

We could simplify this a great deal by making run_at a field in RecurringEvent, but I’ve added a separate model in my project because I’d like to log other information of the occurence later on.

Calculate next date for the event

We also need to calculate the date for the next occurence of a recurring event. For this, I use another plain Ruby class in app/services that takes a RecurringEvent instance and returns a date.

One thing I haven’t addressed in this code is timezones, and we will fix that later on in this article.

# app/services/next_recurring_event_date.rb
class NextRecurringEventDate < Struct.new(:recurring_event)
  def calculate
    # TODO: Needs to handle user timezones
    t = Time.zone.now.next_week(day_of_week) + offset.seconds
    [(t-7.days), t].detect(&:future?).utc
  end

  private

  # Convert integer to symbol
  def day_of_week
    Date::DAYNAMES.fetch(recurring_event.day).downcase.to_sym
  end

  def offset
    t, am_pm = recurring_event.time.downcase.split(' ')
    hh, mm = t.split(':')
    am_pm_offset = (am_pm == 'pm' ? 3600*12 : 0)
    offset = Integer(hh)*3600 + Integer(mm)*60
    offset + am_pm_offset
  end
end

Persisting a RecurringEvent

Now that we have everything in place, let’s look at RecurringEvents::Create. Here we just persist a RecurringEvent to the database, and call Events::Schedule service class to create a new instance of Event with the run_at field set correctly.

# app/services/recurring_events/create.rb
class RecurringEvents::Create < Struct.new(:user, :params)
  def call
    recurring_event = user.recurring_events.build(params)
    recurring_event.save &&
      Events::Schedule.new(recurring_event).call
    recurring_event
  end
end

Let’s look at the Events::Schedule class, which does the following:

  • Creates an instance of the Event class and sets the run_at field.
  • Schedules a background job that will run at Event#run_at. This will be handled by the RunEventJob ActiveJob class.
# app/services/events/schedule.rb
class Events::Schedule < Struct.new(:recurring_event)
  def call
    next_date = NextRecurringEventDate.new(recurring_event).calculate
    event = recurring_event.events.build(run_at: next_date)
    event.save && schedule_background_job(event)
  end

  private

  def schedule_background_job(event)
    RunEventJob.set(wait_until: event.run_at)
      .perform_later(event)
  end
end

The RunEventJob should contain the code for whatever should happen at the time, and at the end calls Events::Schedule#call to queue up the next occurence of the event.

class RunEventJob < ApplicationJob
  queue_as :default

  def perform(event)
    # Perform the relevant work here. Example:
    # ReminderMailer.notify(event).deliver_later!

    # Schedule the next occurence of the event
    Events::Schedule.new(event.recurring_event).call
  end
end

Timezone considerations

Remember the timezone problem I mentioned in NextRecurringEventDate#calculate? When a user wants to send a reminder at “Friday 5PM”, they usually mean 5pm on Friday in their timezone. We should make sure that the event time that we calculate uses the correct timezone offset.

In this article I have not mentioned a User model. Let’s assume that we do have such a model, and we have associated RecurringEvent as belonging to a user.

We will also need to be aware of the user timezone. This is another thing that I will not be addressing in this article, but let’s assume that we can access the user’s timezone as recurring_event.user.timezone. (My previous article on setting user timezones during user signup might be useful at this point.)

# app/services/next_recurring_event_date.rb
class NextRecurringEventDate < Struct.new(:recurring_event)
  def calculate
    t = Time.use_zone(recurring_event.user.timezone) {
      Time.zone.now.next_week(day_of_week) + offset.seconds
    }
    [(t-7.days), t].detect(&:future?).utc
  end
end

With that, we now have a working weekly recurring events system. There are many more things to consider, though, and I will list some gotchas below.

Wrapping up

A couple of gotchas you might need to consider if you’re building a similar feature:

  • When you allow users to delete a RecurringEvent, you might see the ActiveJob::DeserializationError in the RunEventJob. This exception could also be caused by other factors, such as the database being down. We need to always retry unless the exception is ActiveRecord::RecordNotFound. The following code achieves that:
class RunEventJob < ApplicationJob
  rescue_from ActiveJob::DeserializationError do |exception|
    exception.original_exception == ActiveRecord::RecordNotFound
  end
  # ...
end
  • When users edit a RecurringEvent, make sure that you update the next Event occurence.

As I mentioned at the start of the article, there are places where I’ve retained code that isn’t needed in a simple example like this one. For instance, I needed a model for each event occurence because I need to add additional information there, but here you could just use a field in the RecurringEvent model.

There are other scenarios in which you might need recurring events, such as “second Saturday of every month”, and this article doesn’t cover such advanced scenarios.

If you’re looking a more complex example, take a look at Martin Fowler’s excellent article [PDF] on modeling recurring events using temporal expressions.

Update: As Swanand points out in the comments, the recurrence rules section in iCalendar specification (RFC 5545) is an interesting read if you’re interested in exploring further.

Hi, I’m Nithin Bekal, a software craftsman with over 7 years of experience in shipping web applications. I mostly use Ruby, but lately have also been exploring Elixir. Co-founder of CrowdStudio.in, and helping organize Rubyconf India. Tweet to me at @nithinbekal.