TIL, 2017-06-20

Singapore Ruby Group Meetup

To ORM or not to ORM - Mykola Kyryk

Web dev is mostly about CRUDs.
ORM/AR: AR wraps a row in a database table or view, encapsulates database access, and adds domain logic on that data.
AR: If you need to get values from multiple tables, what do you do?
PocketMath workflow: customer creates ad campaign, platform receives traffic, then we check if the click can be linked to us.
API endpont: See money spent and clicks per campaign.

{
  campaign_name => campaigns table
  total_budget => campaigns table
  spend => money spent, from stats table
  clicks => clicks, from stats table
}

New requirement, if you need to measure the campaign stats for a user, since campaign belongs_to customer.
You do the map thing, but you will run into the N+1 query.
You can do the join thing customer.campaigns.join(:stats), but you don’t select stats and customer table by default.
You can do the custom select, but it returns more rows.
You can do the custom select with join, using an aggregate as and group.
Moving the query to the scope: you either do the custom SQL as a scope, or you use Arel in the scope.
Result: You are able to select values from 2 tables, db does the heavy lifting, you can customize AR with SQL.
Problem: AR is expensive. The solution is to do the pluck method, the problem is that you can’t call methods defined on an AR class. You need to do the as spend.
So that solves the thing, but new problem. How do you do the same thing. You can create an SQL view. Um technically you can actually do an inherits from AR::Base, but you can’t do inserts. You can only do wheres.

Summary

Prefer pure SQL to Arel, because you never change databases anyway (and if you do change, db-specific things like groups will not work.
ORM for third party code, since it returns AR::Relation. You can use things like kaminari, paper_trail
SQL database view comes with db:schema:load.

Releasing your First Gem - Peter

Need to do mail migrations, previously this was a shell script.
Swiss Army Knife for SMTP

Peter’s Rules

Google it.
You will write code so ugly, not even a mother can love it.
Nobody will read it anyway.

Confess Your MongoDB Opinions - Emily Stolfo

Postgres has JSON support so why Mongo? You can index inside the keys, but you can’t aggregate inside the keys.
All data is relational, so what data do you gain?
MongoDB good points: aggregation?
Rails people think in models and database tables.
Aggregating thing is done in C++, faster than map reduce JS.