Ruby DCamp 2010 Session 2: AREL
Snuggs
Edger F. Codd - the inventor of the relational database
This is about using Arel, instead of using Arel like ActiveRecord. (harnessing the power of the relational algebra)
What is relational algebra?
- Operations: select, rename, union, project, product, difference, join, intesection, aggregate
- Closure: A set is closed IF the result of the operation on the set products a member of the set
- e.g. Addition is closed for integers, since any integer added to another integer produces an integer (e.g., 1 + 1 = 2).
- BUT division is NOT closed for integers, since one integer divided by another integer is not always an integer (e.g., 3 / 2 = 1.5, not an integer)
There are a LOT more relationships you can use in Arel besides ‘where’
Arel gives you a Ruby implementation that matches very closely the relational algebra represented in SQL
The COOL thing about Arel is that it will not execute your relationships until you need to use them, i.e., it is an in-memory engine.
- That means that you do not hit the database unless (or until) you need to.
ActiveRecord::Relation != Arel::Relation
- Arel uses a lot of things that ActiveRecord does not use. ActiveRecord is stuck with a lot of legacy stuff. If you can, having Arel as a core gives you a lot more power for manipulating your data.
Arel is database-agnostic!
Arel::Relation::Operable::Writable is the class that handles side effects through Arel (side effects are what make programming languages useful)
Arel Algebra is a Query Evaluator, and GenericCompiler is a Query Optimizer
Arel runs with many different engines: Memory, SQL, and … whatever you can make it.
What is a set? A set is a collection of things.
- hi-hat is an element of Cymbals
- U is the universe of all sets
- 0 is the empty (or null) set
- T is a set relation, which is a subset of U
- If T is vowels, then (T) = {a,e,i,o,u}
- a is in the set T
- z is NOT in the set T
A tuple is a heterogeneous set of sets, like a row in a database. It is any collection of anything.
#How to use Arel without ActiveRecord!
require 'rubygems'
require 'arel'
class YourClassName
include Arel::Relation
#Your code here
t = Arel::Table(:cars) # this is a new set relation
t.attributes # returns a header, this contains all our attributes
t[:tires] # Adds the tires attribute to cars
t.project(t[:tires]) # this creates a projection that will get the tires attribute from the cars table.
four_tires = t.where(:tires => 4) # this returns a Table where tires equals 4
sedans = four_tires.where(:type => 'sedan') # nothing is executes on the database yet, it is 'lazy'
endA GREAT way to learn Arel is to study the Arel tests. They are very verbose and easy to understand (written in RSpec).
