MySQL Triggers w/Rails

I recently incorporated db-triggers with a Rails app to maintain some counts that were otherwise fairly expensive to retrieve.  Rails wasn’t super-pumped about the idea (what with the “keep all the logic in the app” approach and all), but sometimes… you know… you know better than your framework.

Some things I was aiming for:

  1. Set them up with normal migrations.
  2. Test them with the normal test suite/normal fixtures.
  3. Make recovery/reset simple for when the table (inevitably) is somehow out of sync.

The “frequent counts” table

I’ll have multiple counts but not TOO many — enough that I don’t want to have a column per count but not enough that I mind using “LIKE” to lookup patterns, so my table has: id, code, current_count.

Code will be a unique key (important later) and be formatted like “style_ABC_size_456”.

So, when a new item is added it’ll be associated with a style and some sizes – each combination will either need to be setup (with a current_count = 1) or an existing combo will be found and +=1.

The FrequentCount class has the fairly straightforward finders that you’d expect + methods to reset each of the counts that it contains.  The reset methods follow the pattern “reset_frequent_count_COUNT_NAME” -> they clear the existing counts that they maintain before repopulating them.

I also threw in a reset_all method that looks for anything on the class following the “reset_frequent_count_COUNT_NAME” pattern and runs them.

The trigger-SQL

The SQL for creating the triggers will be needed by the migration as well as the test suite.  In fact, the test suite will need to run them somewhat often due to the way the standard tests “prepare” the database.

I ended up throwing it in lib/trigger_sql.rb.  Methods there are named with the pattern “sql_for_TABLE_OPERATION_TRIGGER_NAME” ex: sql_for_items_insert_style_and_size

Many of the triggers could not rely on pre-existing rows.  i.e. a new style/size combination needs to INSERT where an existing combo could update ( +=1 ).  To get around this, I relied on the unique key setup earlier on the “code” field for the frequent counts table.  <– that allowed me to lean on insert statements with “ON DUPLICATE KEY” clauses with update statements.  Something like this…
create trigger items_insert_style_and_size after insert on items
for each row
insert into frequent_counts(code, current_count)
values (concat(‘style_’,, ‘_size_’, new.size), 1)
on duplicate key update current_count = current_count + 1;
The Migration

I’ve already given away most of the fun stuff about the migration.  It just needs to run through the triggers that are being setup at this specific time, doing things like:
and then make sure to populate it all (with that reset_all) method when we’re done. <– next time out I may want to call specific methods to reset just the ones I care about but this first time, I can just do the whole table.

Testing with Fixtures
Rails goes a little too far when running the default test tasks for us – it ends up nuking the triggers on us, but not to fear: it’s a quick hack in the Rakefile.
I’m going to spare you some details (drop me a line if you want them) but I basically overrode the db:test:prepare method to call a special version of the clone_structure task.  My version has a dependent task that does:
# find methods that follow our pattern of “methods providing trigger sql” and execute the contents of each{ |m| m =~ /sql_for_.+/ }.each do |method_name|
As you see there, it’s leaning on that naming convention “sql_for_TABLE_OPERATION_TRIGGER_NAME” to find the sql to (re)apply.
That’s it!
Migrations set them up and share the code to do so with the fixtures that can repeat the tests whenever we need.  Those reset methods also come in handy not only for the initial population (by the migration) but we can call them manually should we need them.

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Filed under deployment, rails, Ruby, SQL, Test Driven Development (TDD)

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