rails--rails/README.markdown

# Arel [![Build Status](https://secure.travis-ci.org/rails/arel.png)](http://travis-ci.org/rails/arel) [![Dependency Status](https://gemnasium.com/rails/arel.png)](https://gemnasium.com/rails/arel)

* http://github.com/rails/arel

## DESCRIPTION

Arel is a SQL AST manager for Ruby. It

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

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[:user_id]))
# => 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[:name])
# => 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[:age].lt(25))
```

Of course, many of the operators take multiple arguments, so the last example can be written more tersely:

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

The `OR` operator works like this:

```ruby
users.where(users[:name].eq('bob').or(users[:age].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)
products.columns
# => [products[:id], products[:name], products[:price], products[:currency_id]]

currency_rates = Arel::Table.new(:currency_rates)
currency_rates.columns
# => [currency_rates[:from_id], currency_rates[:to_id], currency_rates[:date], currency_rates[: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
comments.columns
# => [comments[:id], comments[:body], comments[: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](http://opensource.org/licenses/MIT).