arel, how to join

Question

Given

class Category < ActiveRecord::Base
  has_many :products, :order => 'name ASC'
end

Using the Rails 3 stack, how can I query for all categories that 'have' products?

Answer 1

Category.joins(:products).select("distinct categories.*").all

Answer 2

In ARel (NOT ActiveRecord) we will do the following:

p = Arel::Table.new :products    # Base Rel-var
c = Arel::Table.new :categories  # Base Rel-var

predicate = p[:category_id].eq( c[:id] ) # for equality predicate

p.join(c)                   # Natural join
  .on( predicate )          # Equi-Join
  .group( p[:category_id] ) # Grouping expression to get distinct categories
  .project( c[:id] )        # Project the distinct category IDs of the derived set.

Answer 3

Another, simpler, approach is to use the ActiveRecord query interface's join in conjunction with ARel for the conditional statement:

joins(:user)
.where(User.arel_table[:name].matches("%#{query}%"))

Generates the following sql in sqlite3:

"SELECT \"patients\".* FROM \"patients\" INNER JOIN \"users\" ON \"users\".\"id\" = \"patients\".\"user_id\" WHERE (\"users\".\"name\" LIKE '%query%')"

And the following sql in postgres (notice the ILIKE):

"SELECT \"patients\".* FROM \"patients\" INNER JOIN \"users\" ON \"users\".\"id\" = \"patients\".\"user_id\" WHERE (\"users\".\"name\" ILIKE '%query%')"

This allows you to join with simplicity, but still get the abstraction of the ARel matcher to your RDBMS.