Time in Ruby and ActiveRecord

I recently lost about a day of work trying to figure what the hell was going on when I was working with an ActiveRecord model that had a time type column. In the hopes that I will not loose such time again and that this post can help others avoid such lost time, I want to lay out my investigation into time in Ruby and ActiveRecord.

Let’s start with Ruby. The documentation for Ruby 2.4.1 describes the Time class like so:

Time is an abstraction of dates and times. Time is stored internally as the number of seconds with fraction since the Epoch, January 1, 1970 00:00 UTC.

In short, an instance of the Time class represents an exact moment in the history of our world (down to the nanosecond), a moment that happened on a particular date and at a particular time. The Ruby documentation offers the following example for creating a new instance of Time where we set the year, month, day, hour, minute, and second: Time.new(2002, 10, 31, 2, 2, 2). Those are the elements (plus the timezone) that compose an instance of Time.

Alright, now what about time in ActiveRecord? In ActiveRecord you can specify that a database column is of type time (e.g. create_table :foos { |t| t.time :column }). Now, if you are anything like me, you don’t have the column type in your application, so you probably don’t know much about what it is or how it works. So, what’s the first thing I do when I’m dealing with a new concept or problem? I start poking at it.

I had a Rails application with a test database, so I opened a console in the test environment and starting poking. I’m working with an instance of the Foo class (defined in this context as foo) that has a column called time that is, you guessed it, of type time:

> foo.time
=> Sat, 01 Jan 2000 01:00:00 UTC +00:00
> foo.time.class
=> ActiveSupport::TimeWithZone
> foo.time
=> Sat, 01 Jan 2000 01:00:00 UTC +00:00
> foo.time.class
=> ActiveSupport::TimeWithZone

The first thing I want to do is look at a value from the time column. I find that ActiveRecord returns an instance of the ActiveSupport::TimeWithZone class, which is a wrapper around Ruby’s Time class.¹ This makes sense; a column of database type time stores an instance of the Ruby Time class, or so I think. The next thing I do is start playing with this time column:

> foo.time = 1.day.from_now.to_time
=> 2017-11-17 15:08:54 -0500
> foo.time
=> Sat, 01 Jan 2000 20:08:54 UTC +00:00
> foo.time = 1.day.from_now.to_time
=> 2017-11-17 15:08:54 -0500
> foo.time
=> Sat, 01 Jan 2000 20:08:54 UTC +00:00

Hrm. All of the date bits about our time disappeared… What in the hell is going on? As any sane developer would when faced with a situation that doesn’t make sense, I start Googling: “activerecord time column”, “active record column types”, etc. I can’t find official documentation anywhere for what the hell a time database column is. Luckily, there are other other sources of unofficial documentation (i.e. StackOverflow). In one I find this:

• Time:
  • Stores only a time (hours, minutes, seconds)

In another I found these images:

<%= image_tag ‘activerecord-column-types-1.png’ %> <%= image_tag ‘activerecord-column-types-2.png’ %>

In my test database, I’m using SQlite, so this column is being stored in the actual database as a SQLite datetime object. However, the first answer says that a column of this type represents only the combination of hour, minute, and second. Ok. Well, still, what’s going on here?

The short answer is that Ruby has no class to represent only the combinarion of hour, minute, and second. While Date represents year, month, and day, and Time represents year, month, day, hour, minute, and second (I will write more about Time and DateTime at a later date), there is no class for simply hour, minute, and second. So, what does ActiveRecord do? The only thing it really can do, use the Time class. But, in order to ensure that the year, month, and day are meaningless, ActiveRecord always forces the year, month, and day of any value set for a time type column to be 2000-01-01.

This is why our date information disappeared in the above example. When ActiveRecord casts the value passed in (1.day.from_now.to_time), it resets the date portion. This can be seen by inspecting the time_before_type_cast value:

> foo.time = 1.day.from_now.to_time
=> 2017-11-17 15:08:54 -0500
> foo.time
=> Sat, 01 Jan 2000 20:08:54 UTC +00:00
> foo.time_before_type_cast
=> 2017-11-17 15:08:54 -0500
> foo.time = 1.day.from_now.to_time
=> 2017-11-17 15:08:54 -0500
> foo.time
=> Sat, 01 Jan 2000 20:08:54 UTC +00:00
> foo.time_before_type_cast
=> 2017-11-17 15:08:54 -0500

So, in short, the ActiveRecord time column type, while it does return an instance of the Ruby Time class, does not represent the same kind of object. Ruby Time represents year, month, day, hour, minute, and second. ActiveRecord time represents hour, minute, and second.