AR: take precision into count when assigning a value to timestamp attribute

[bogdan]

The Problem

Consider this following on MySQL (that have 0 precision on datetime by default):

d1 = Developer.create!(name: 'bogdan')
d2 = Developer.find(d1.id)
d1.created_at.usec # => 837728
d2.created_at.usec # => 0
# It results in a madness:
d1.created_at == d2.created_at # => false
d1.cache_key == d2.cache_key # => false

Rails 4.1.10 and master are affected. Not sure about others.

Timestamp column can have less precision than ruby timestamp
In result in how big a fraction of a second can be stored in the
database.

If the precision is low enough, (mysql default is 0 so it is always low
enough by default) the value changes when model is reloaded from the
database.

The Patch

In order to fix that we just use a precision of a column when assigning a value to the timestamp attribute as time object (not as string - that seems working).

Fixing tests

When I've applied a patch, the new problem raised: existing tests are written incorrectly for mysql that have 0 precision by default.
There are many tests that check that updated_at changes on particular model. But it is only true for ruby level but not in the database. This patch reveals the problem.

In order to make them pass I've set the precision for tables that are used to check if updated_at is changing big enough and consistent for all databases.

Further problems

datetime column type works differently in different databases now because if different default precision. It results in different behaviour of the same AR schema on different databases.

It should be consistent. It can be done by setting up a precision option of add_column to some value all the time and disallowing it to be nil.

[rafaelfranca]

Tests are broken because datetime precision is not supported at the version of mysql that we have in our servers. This make me wonder, would this patch work on old MySQL versions?

[bogdan]

@rafaelfranca yes it will: at the rails level, unsupported precision works like precision # => 0. It means that all seconds fractions will be removed.

The fact that older mysql don't support precision raises the following problem:

  def test_cache_key_changes_when_child_touched
    owner = owners(:blackbeard)
    pet   = pets(:parrot)

    owner.update_column :updated_at, Time.current
    key = owner.cache_key

    assert pet.touch
    assert_not_equal key, owner.reload.cache_key
  end

It will only work stable if there will be at least 1 second between the creation of owner and touch of pet. It can be solved by putting sleep(1) there or by marking as pending on old versions of mysql.

I've put sleep(1) initially but it didn't work because too many tests would require it and they end up being very slow.

I'll think more how we can solve it. If you have an idea, please share with me.

[rafaelfranca]

cc @sgrif

[bogdan]

Hope I've fixed the build on mysql5.6: I made it differently with travel so that tests don't relay on ruby-is-slow.

Can someone advice how to go from PR to corresponding CI build? Find it here: https://travis-ci.org/rails/rails/pull_requests manually seems impossible.

[bogdan]

@sgrif @rafaelfranca I understand the work load of rails core team, but this is regression problem from 4.0 to 4.1 and we kind of expect a feedback sooner than later. Can you please merge this in and backport to 4.1 or point out why it can not be done?

[sgrif]

Forcing a Travis build.

[sgrif]

I'm traveling and don't have my computer right now but I'll take care of this next chance I get.

[bogdan]

@sgrif if you have back to work, it would be cool to receive another review here

[sgrif]

Are you Github stalking me? Taking a look now.

[sgrif]

I find this test hard to follow, and I don't think this is relevant to the behavior of AR::Timestamp. Can we move this to a separate test in something like test/cases/type/date_time_test.rb, and have it look something like this?

model = Model.new(date_time_attr_that_isnt_auto_touched: Time.now)
assert_equal model.date_time_attr, model.tap(&:save).reload.date_time_attr

[bogdan]

I don't see any test like that inside am/test/cases/type/ that uses Model. They all just instantiate the AM::Type::* object and call #cast method there.

Also, testing this feature in AM will not cover this change:
https://github.com/rails/rails/pull/20317/files#diff-2695559f74d901580d9c9314a9f4aff7R14
that caused postgresql specific bug.

I would stick with test we have know. I will only change it with use of #reload.

[sgrif]

If it uses a model, create a test in the active record namespace.

[sgrif]

There are already several tests of this nature in activerecord/test/cases/type

[sgrif]

A few nitpicks. This will need to be rebased. The relevant classes have been moved to Active Model.

[bogdan]

You are right about that test. I didn't get what you meant from the first try.

So:

• Renamed method to apply_seconds_precision. Feel like it is the best.
• Reworked test to be in proper place and don't depend on autoupdated time stamps.