Database Design for Recurring Events with Exceptions

Database design for recurring events with exceptions

Use iCalendar RRules and ExDates

If it's a recurring event, just store the start/end datetimes and RRules and ExDates for the event.

Use a Materialized View to pre-calculate upcoming actual events, say for the next 30 days or 365 days.

As you are using Postgres, you can use existing python, perl, or javascript RRule libraries (such as dateutil) inside pg function for calculating future events based on the rrules and exdates

UPDATE: check out pg_rrule extension: https://github.com/petropavel13/pg_rrule

Recurring events attendance database design

You shouldn't be trying to avoid recording instances of recurring events. That's what databases are for.

You need one recurringevent table which has a single record per recurring event, containing recurrence rules. A very simple one would be as follows:

id
event_name
start_date
end_date
DaysBetween
RecordCreateDateTime
RecordCreatedBy

Note that modelling recurring events against a calendar is complicated... once per week, once per month, second Tuesday of every month.

You need another table which records "instances" of that recurring event say events. It has a record for every actual event, and a reference back to recurringevent. Creating, editing, deleting the parent recurringevent record generates these records automatically.

Records in this table may or may not be generated from recurring events. You might just have a one-off event in which case you have one events record with no parent recurringevent.

id
recurringevent_id   (null if this is a once off)
date
event_name
RecordCreateDateTime
RecordCreatedBy

Then you need an eventattendance table with one row per attending person which refers back to events

id
event_id
user_id
event_feedback
RecordCreateDateTime
RecordCreatedBy    

Right now your events table is trying to be two things. It should only represent an actual "event" and should not hold recurrence information.

Calendar Recurring/Repeating Events - Best Storage Method

Storing "Simple" Repeating Patterns

For my PHP/MySQL based calendar, I wanted to store repeating/recurring event information as efficiently as possibly. I didn't want to have a large number of rows, and I wanted to easily lookup all events that would take place on a specific date.

The method below is great at storing repeating information that occurs at regular intervals, such as every day, every n days, every week, every month every year, etc etc. This includes every Tuesday and Thursday type patterns as well, because they are stored separately as every week starting on a Tuesday and every week starting on a Thursday.

Assuming I have two tables, one called events like this:

ID    NAME
1     Sample Event
2     Another Event

And a table called events_meta like this:

ID    event_id      meta_key           meta_value
1     1             repeat_start       1299132000
2     1             repeat_interval_1  432000

With repeat_start being a date with no time as a unix timestamp, and repeat_interval an amount in seconds between intervals (432000 is 5 days).

repeat_interval_1 goes with repeat_start of the ID 1. So if I have an event that repeats every Tuesday and every Thursday, the repeat_interval would be 604800 (7 days), and there would be 2 repeat_starts and 2 repeat_intervals. The table would look like this:

ID    event_id      meta_key           meta_value
1     1             repeat_start       1298959200 -- This is for the Tuesday repeat
2     1             repeat_interval_1  604800
3     1             repeat_start       1299132000 -- This is for the Thursday repeat
4     1             repeat_interval_3  604800
5     2             repeat_start       1299132000
6     2             repeat_interval_5  1          -- Using 1 as a value gives us an event that only happens once

Then, if you have a calendar that loops through every day, grabbing the events for the day it's at, the query would look like this:

SELECT EV.*
FROM `events` EV
RIGHT JOIN `events_meta` EM1 ON EM1.`event_id` = EV.`id`
RIGHT JOIN `events_meta` EM2 ON EM2.`meta_key` = CONCAT( 'repeat_interval_', EM1.`id` )
WHERE EM1.meta_key = 'repeat_start'
    AND (
        ( CASE ( 1299132000 - EM1.`meta_value` )
            WHEN 0
              THEN 1
            ELSE ( 1299132000 - EM1.`meta_value` )
          END
        ) / EM2.`meta_value`
    ) = 1
LIMIT 0 , 30

Replacing {current_timestamp} with the unix timestamp for the current date (Minus the time, so the hour, minute and second values would be set to 0).

Hopefully this will help somebody else too!

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Storing "Complex" Repeating Patterns

This method is better suited for storing complex patterns such as

Event A repeats every month on the 3rd of the month starting on March 3, 2011

or

Event A repeats Friday of the 2nd week of the month starting on March 11, 2011

I'd recommend combining this with the above system for the most flexibility. The tables for this should like like:

ID    NAME
1     Sample Event
2     Another Event

And a table called events_meta like this:

ID    event_id      meta_key           meta_value
1     1             repeat_start       1299132000 -- March 3rd, 2011
2     1             repeat_year_1      *
3     1             repeat_month_1     *
4     1             repeat_week_im_1   2
5     1             repeat_weekday_1   6

repeat_week_im represents the week of the current month, which could be between 1 and 5 potentially. repeat_weekday in the day of the week, 1-7.

Now assuming you are looping through the days/weeks to create a month view in your calendar, you could compose a query like this:

SELECT EV . *
FROM `events` AS EV
JOIN `events_meta` EM1 ON EM1.event_id = EV.id
AND EM1.meta_key = 'repeat_start'
LEFT JOIN `events_meta` EM2 ON EM2.meta_key = CONCAT( 'repeat_year_', EM1.id )
LEFT JOIN `events_meta` EM3 ON EM3.meta_key = CONCAT( 'repeat_month_', EM1.id )
LEFT JOIN `events_meta` EM4 ON EM4.meta_key = CONCAT( 'repeat_week_im_', EM1.id )
LEFT JOIN `events_meta` EM5 ON EM5.meta_key = CONCAT( 'repeat_weekday_', EM1.id )
WHERE (
  EM2.meta_value =2011
  OR EM2.meta_value = '*'
)
AND (
  EM3.meta_value =4
  OR EM3.meta_value = '*'
)
AND (
  EM4.meta_value =2
  OR EM4.meta_value = '*'
)
AND (
  EM5.meta_value =6
  OR EM5.meta_value = '*'
)
AND EM1.meta_value >= {current_timestamp}
LIMIT 0 , 30

This combined with the above method could be combined to cover most repeating/recurring event patterns. If I've missed anything please leave a comment.

What's the best way to model recurring events in a calendar application?

I would use a 'link' concept for all future recurring events. They are dynamically displayed in the calendar and link back to a single reference object. When events have taken place the link is broken and the event becomes a standalone instance. If you attempt to edit a recurring event then prompt to change all future items (i.e. change single linked reference) or change just that instance (in which case convert this to a standalone instance and then make change). The latter cased is slightly problematic as you need to keep track in your recurring list of all future events that were converted to single instance. But, this is entirely do-able.

So, in essence, have 2 classes of events - single instances and recurring events.

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