Arel
¶ ↑
DESCRIPTION¶ ↑
Arel
Really Exasperates Logicians
Arel
is a SQL AST manager for Ruby. It
-
simplifies the generation of complex SQL queries, and
-
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 and perform connection pooling, quoting, and 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)
Bitwise operators &
, |
, ^
, <<
, >>
:
users.where((users[:bitmap] & 16).gt(0)).project(Arel.sql('*')) # => SELECT * FROM "users" WHERE ("users"."bitmap" & 16) > 0 users.where((users[:bitmap] | 16).gt(0)).project(Arel.sql('*')) # => SELECT * FROM "users" WHERE ("users"."bitmap" | 16) > 0 users.where((users[:bitmap] ^ 16).gt(0)).project(Arel.sql('*')) # => SELECT * FROM "users" WHERE ("users"."bitmap" ^ 16) > 0 users.where((users[:bitmap] << 1).gt(0)).project(Arel.sql('*')) # => SELECT * FROM "users" WHERE ("users"."bitmap" << 1) > 0 users.where((users[:bitmap] >> 1).gt(0)).project(Arel.sql('*')) # => SELECT * FROM "users" WHERE ("users"."bitmap" >> 1) > 0 users.where((~ users[:bitmap]).gt(0)).project(Arel.sql('*')) # => SELECT * FROM "users" WHERE ~ "users"."bitmap" > 0
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. Here is an example of the DISTINCT
operator:
posts = Arel::Table.new(:posts) posts.project(posts[:title]) posts.distinct posts.to_sql # => 'SELECT DISTINCT "posts"."title" FROM "posts"'
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 Advanced 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¶ ↑
Alias¶ ↑
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.
CTE¶ ↑
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
Write SQL strings¶ ↑
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 the 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.