Using Arel to Compose SQL Queries
Arel is a library that was introduced in Rails 3 for use in constructing SQL queries. Every time you pass a hash to where, it goes through Arel eventually. Rails exposes this with a public API that we can hook into when we need to build a more complex query.
Related cards:
How to tell ActiveRecord how to preload associations (either JOINs or separate queries)
Remember why preloading associations "randomly" uses joined tables or multiple queries?
If you don't like the cleverness of thi...
How to explain SQL statements via ActiveRecord
ActiveRecord offers an explain method similar to using EXPLAIN SQL statements on the database.
However, this approach will explain all queries for the given scope which may include joins or includes.
Output will resemble your database's ...
How to load an SQL dump from a migration
If you want to load an SQL dump from an ActiveRecord migration, you might find this to be harder than you thought. While you can call ActiveRecord::Base.connection.execute(sql) to execute arbitrary SQL commands, the MySQL connection is configure...
Timeouts for long-running SQL queries
While the main goal always is to prevent long-running queries in the first place, automatic timeouts can serve as a safety net to terminate problematic queries automatically if a set time limit is exceeded. This prevents single queries from taking...
Howto use ActiveRecord preload with plain SQL inner joins
Like you know from "How to tell ActiveRecord how to preload associations (either JOINs or separate queries)", you can tell ActiveRecord explicitly if it should use a LEFT OUTER JOIN or a separate query to preload associations....
Rails: Keeping structure.sql stable between developers
Why Rails has multiple schema formats
When you run migrations, Rails will write your current database schema into db/schema.rb. This file allows to reset the database schema without running migrations, by running rails db:schema:load.
The ...
Measuring sql query time of a piece of code using ActiveSupport::Notifications
ActiveSupport::Notifications provides an instrumentation API for Ruby. It is used throughout rails to publish instrumentation events that include information about each part of a request/response cycle.
Have a look at your application log file -...
Advice: Reduce scopes with joins to simple IN-queries
In theory you can take any scope and extend it with additional joins or conditions. We call this chaining scopes.
In practice chaining becomes problematic when scope chains grow more complex. In particular **having JOINs in your scope will re...
Rails: Talking to the database without instantiating ActiveRecord objects
Instantiating ActiveRecord objects comes expensive. To speed up things, you can choose a more direct way to talk to your database: the [ActiveRecord::ConnectionAdapters::DatabaseStatements](http://api.rubyonrails.org/classes/ActiveRecord/Connect...
How to use pessimistic row locks with ActiveRecord
When requests arrive at the application servers simultaneously, weird things can happen. Sometimes, this can also happen if a user double-clicks on a button, for example.
This often leads to problems, as two object instances are modified in paral...