Re: Testing Technique when using a DB - Mailing list pgsql-general

On Mar 12, 2013, at 8:42 AM, Perry Smith <pedzsan@gmail.com> wrote:

I tried posting this from Google Groups but I did not see it come through
after an hour so this may be a duplicate message for some.

The current testing technique for things like Ruby On Rails has three
choices but all of the choices will not work in my case.

The first choice is "truncate" which starts a transaction before each test.


I think you mean "transactional tests" here, not "truncate". While the test
database is truncated once at the start of all the tests, each test runs
inside its own transaction which will never be committed.

 If the testing is within the same process, this works and a roll back
restores the DB.  But if the testing involves two processes, then the test
data entered by the test rig can not be seen by the "system under test"
(SUT).  With Rails, there are times when this is needed.  The test rig
drives a browser which calls into a Rails application.  There are
"dangerous" ways to still use this method but each has various down falls
or risks.


IMO, you don't want to use transactional tests. When you do, ActiveRecord
will use savepoints to mimic transactions. This means now() will never
change during the tests, deferred constraints/triggers won't work, other
processes can't see the data, etc.  The system is behaving differently in
the test environment than in the real one, which is bad.

If you are treating the database as a really dumb store of data, then you
may want to use transactional tests. But be aware of the caveats.


The other two choices are delete and truncate which both end up with an
empty database just after each test.  This prevents any test data that is
already in the database from being used after the first test.  Note that a
"test run" will run through a sequence of tests (usually quite a few).

All of these are fairly fast with each one being "faster" under different
conditions (according to users).

Generally, this pushes the Rails community to have either "fixtures" or
"factories".  Both are ok solutions but both also have problems.  In my
case, I have a dozen or so tables all with very tight constraints and
creating either fixtures or factories is very troublesome.  Also, I have a
real database with real data in production and it seems foolish not to take
advantage of the knowledge contained within that database.  By "knowledge"
I mean the particular values and weirdness within the data that a factory
or a fixture might not realize.

One choice would be to create the database, use it, and then drop it for
each test.  I would create the database from a template that already has
data taken from the production database (and probably trimmed down to a
small subset of it).  This requires some crafty dancing in the Rails set up
since it likes to just attach to a database and run but it could be done.
 From first blush, this sounds like it would be really slow but may be not.

The other choice would be to somehow copy the data to temporary tables
before the test run and then copy it back.  The advantage to this is it is
not very PostgreSQL specific.  Indeed, if the template database is already
set up, then only one copy would be needed at the start of the test.

The other thought I had is if there is some type of "leaky" transaction.  A
transaction where another process can see the data but the roll back would
still work and be effective.  Essentially I'm asking if all the protections
a database offers could be dropped... but I thought I'd see if that was
possible.

The other thought is perhaps there is a "snap shot" type concept.  I don't
see it in the list of SQL commands.  A "snap shot" would do exactly what it
sounds like.  It would take a snap shot and save it somehow.  Then a
"restore to snap shot" would restore the DB back to that state.


This would be super super super awesome, but it doesn't exist as far as I
know. This would be a "permanent snapshot" that could be easily and quickly
restored.

I wonder if it would be possible to make an extension that made this easy
to do.


I thought this group might suggest other ideas and either nuke the really
bad ideas or promote the plausible ideas I've mentioned above.


Make sure to look at database_cleaner if you haven't yet. Also this may be
interesting: https://github.com/bmabey/database_cleaner/issues/80

Personally, I use database_cleaner's delete method, plus I load a SQL file
at the beginning of each test.



Sorry for the long post.  I appreciate your thoughts.

Perry


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