rails--rails/README.markdown

Arel

• http://github.com/rails/arel

DESCRIPTION

Arel Really Exasperates Logicians

Arel is a SQL AST manager for Ruby. It

1. Simplifies the generation of complex SQL queries
2. Adapts to various RDBMSes

It is intended to be a framework framework; that is, you can build your own ORM with it, focusing on innovative object and collection modeling as opposed to database compatibility and query generation.

Status

For the moment, Arel uses Active Record's connection adapters to connect to the various engines, connection pooling, perform quoting, and do type conversion.

A Gentle Introduction

Generating a query with Arel is simple. For example, in order to produce

SELECT * FROM users

you construct a table relation and convert it to sql:

users = Arel::Table.new(:users)
query = users.project(Arel.sql('*'))
query.to_sql

More Sophisticated Queries

Here is a whirlwind tour through the most common SQL operators. These will probably cover 80% of all interaction with the database.

First is the 'restriction' operator, where:

users.where(users[:name].eq('amy'))
# => SELECT * FROM users WHERE users.name = 'amy'

What would, in SQL, be part of the SELECT clause is called in Arel a projection:

users.project(users[:id])
# => SELECT users.id FROM users

Comparison operators =, !=, <, >, <=, >=, IN:

users.where(users[:age].eq(10)).project(Arel.sql('*')) # => SELECT * FROM "users"  WHERE "users"."age" = 10
users.where(users[:age].not_eq(10)).project(Arel.sql('*')) # => SELECT * FROM "users"  WHERE "users"."age" != 10
users.where(users[:age].lt(10)).project(Arel.sql('*')) # => SELECT * FROM "users"  WHERE "users"."age" < 10
users.where(users[:age].gt(10)).project(Arel.sql('*')) # => SELECT * FROM "users"  WHERE "users"."age" > 10
users.where(users[:age].lteq(10)).project(Arel.sql('*')) # => SELECT * FROM "users"  WHERE "users"."age" <= 10
users.where(users[:age].gteq(10)).project(Arel.sql('*')) # => SELECT * FROM "users"  WHERE "users"."age" >= 10
users.where(users[:age].in([20, 16, 17])).project(Arel.sql('*')) # => SELECT * FROM "users"  WHERE "users"."age" IN (20, 16, 17)

Joins resemble SQL strongly:

users.join(photos).on(users[:id].eq(photos[:user_id]))
# => SELECT * FROM users INNER JOIN photos ON users.id = photos.user_id

Left Joins

users.join(photos, Arel::Nodes::OuterJoin).on(users[:id].eq(photos[:user_id]))
# => SELECT FROM users LEFT OUTER JOIN photos ON users.id = photos.user_id

What are called LIMIT and OFFSET in SQL are called take and skip in Arel:

users.take(5) # => SELECT * FROM users LIMIT 5
users.skip(4) # => SELECT * FROM users OFFSET 4

GROUP BY is called group:

users.project(users[:name]).group(users[:name])
# => SELECT users.name FROM users GROUP BY users.name

The best property of arel is its "composability", or closure under all operations. For example, to restrict AND project, just "chain" the method invocations:

users                                 \
  .where(users[:name].eq('amy'))      \
  .project(users[:id])                \
# => SELECT users.id FROM users WHERE users.name = 'amy'

All operators are chainable in this way, and they are chainable any number of times, in any order.

users.where(users[:name].eq('bob')).where(users[:age].lt(25))

The OR operator works like this:

users.where(users[:name].eq('bob').or(users[:age].lt(25)))

The AND operator behaves similarly.

Aggregate functions AVG, SUM, COUNT, MIN, MAX, HAVING:

photos.group(photos[:user_id]).having(photos[:id].count.gt(5)) # => SELECT FROM photos GROUP BY photos.user_id HAVING COUNT(photos.id) > 5
users.project(users[:age].sum) # => SELECT SUM(users.age) FROM users
users.project(users[:age].average) # => SELECT AVG(users.age) FROM users
users.project(users[:age].maximum) # => SELECT MAX(users.age) FROM users
users.project(users[:age].minimum) # => SELECT MIN(users.age) FROM users
users.project(users[:age].count) # => SELECT COUNT(users.age) FROM users

Aliasing Aggregate Functions:

users.project(users[:age].average.as("mean_age")) # => SELECT AVG(users.age) AS mean_age FROM users

The Crazy Features

The examples above are fairly simple and other libraries match or come close to matching the expressiveness of Arel (e.g., Sequel in Ruby).

Inline math operations

Suppose we have a table products with prices in different currencies. And we have a table currency_rates, of constantly changing currency rates. In Arel:

products = Arel::Table.new(:products)
# Attributes: [:id, :name, :price, :currency_id]

currency_rates = Arel::Table.new(:currency_rates)
# Attributes: [:from_id, :to_id, :date, :rate]

Now, to order products by price in user preferred currency simply call:

products.
  join(:currency_rates).on(products[:currency_id].eq(currency_rates[:from_id])).
  where(currency_rates[:to_id].eq(user_preferred_currency), currency_rates[:date].eq(Date.today)).
  order(products[:price] * currency_rates[:rate])

Complex Joins

Where Arel really shines is in its ability to handle complex joins and aggregations. As a first example, let's consider an "adjacency list", a tree represented in a table. Suppose we have a table comments, representing a threaded discussion:

comments = Arel::Table.new(:comments)

And this table has the following attributes:

# [:id, :body, :parent_id]

The parent_id column is a foreign key from the comments table to itself. Joining a table to itself requires aliasing in SQL. This aliasing can be handled from Arel as below:

replies = comments.alias
comments_with_replies = \
  comments.join(replies).on(replies[:parent_id].eq(comments[:id])).where(comments[:id].eq(1))
# => SELECT * FROM comments INNER JOIN comments AS comments_2 WHERE comments_2.parent_id = comments.id AND comments.id = 1

This will return the reply for the first comment.

Common Table Expressions(CTE) support via:

Create a CTE

cte_table = Arel::Table.new(:cte_table)
composed_cte = Arel::Nodes::As.new(cte_table, photos.where(photos[:created_at].gt(Date.current)))

Use the created CTE:

users.
  join(cte_table).on(users[:id].eq(cte_table[:user_id])).
  project(users[:id], cte_table[:click].sum).
  with(composed_cte)

# => WITH cte_table AS (SELECT FROM photos  WHERE photos.created_at > '2014-05-02') SELECT users.id, SUM(cte_table.click) FROM users INNER JOIN cte_table ON users.id = cte_table.user_id

When your query is too complex for Arel, you can use Arel::SqlLiteral:

photo_clicks = Arel::Nodes::SqlLiteral.new(<<-SQL
    CASE WHEN condition1 THEN calculation1
    WHEN condition2 THEN calculation2
    WHEN condition3 THEN calculation3
    ELSE default_calculation END
SQL
)
photos.project(photo_clicks.as("photo_clicks"))
# => SELECT CASE WHEN condition1 THEN calculation1
    WHEN condition2 THEN calculation2
    WHEN condition3 THEN calculation3
    ELSE default_calculation END
 FROM "photos"

Contributing to Arel

Arel is work of many contributors. You're encouraged to submit pull requests, propose features and discuss issues.

See CONTRIBUTING.

License

Arel is released under the MIT License.