Path: | README.markdown |
Last Update: | Sat Feb 23 07:11:49 +0000 2019 |
# Arel [](http://travis-ci.org/rails/arel) [](https://gemnasium.com/rails/arel)
## DESCRIPTION
Arel is a SQL AST manager for Ruby. It
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
```sql SELECT * FROM users ```
you construct a table relation and convert it to sql:
```ruby 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 relational operators. These will probably cover 80% of all interaction with the database.
First is the ‘restriction’ operator, `where`:
```ruby 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`:
```ruby users.project(users[:id]) # => SELECT users.id FROM users ```
Joins resemble SQL strongly:
```ruby users.join(photos).on(users[:id].eq(photos)) # => SELECT * FROM users INNER JOIN photos ON users.id = photos.user_id ```
What are called `LIMIT` and `OFFSET` in SQL are called `take` and `skip` in Arel:
```ruby users.take(5) # => SELECT * FROM users LIMIT 5 users.skip(4) # => SELECT * FROM users OFFSET 4 ```
`GROUP BY` is called `group`:
```ruby users.project(users[:name]).group(users) # => SELECT users.name FROM users GROUP BY users.name ```
The best property of the Relational Algebra is its "composability", or closure under all operations. For example, to restrict AND project, just "chain" the method invocations:
```ruby 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.
```ruby users.where(users[:name].eq('bob')).where(users.lt(25)) ```
The `OR` operator works like this:
```ruby users.where(users[:name].eq('bob').or(users.lt(25))) ```
The `AND` operator behaves similarly.
### 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:
```ruby 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:
```ruby 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 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:
```ruby comments = Arel::Table.new(:comments) ```
And this table has the following attributes:
```ruby # [:id, :body, :parent_id] ```
The `parent_id` column is a foreign key from the `comments` table to itself. Now, joining a table to itself requires aliasing in SQL. In fact, you may alias in Arel as well:
```ruby replies = comments.alias comments_with_replies = \
comments.join(replies).on(replies[:parent_id].eq(comments[:id]))
# => SELECT * FROM comments INNER JOIN comments AS comments_2 WHERE comments_2.parent_id = comments.id ```
This will return the first comment‘s reply‘s body.
### License
Arel is released under the [MIT License](opensource.org/licenses/MIT).