== Sequel: Concise ORM for Ruby

Sequel is an ORM framework for Ruby. Sequel provides thread safety, connection pooling, and a concise DSL for constructing queries and table schemas.

Sequel makes it easy to deal with multiple records without having to break your teeth on SQL.

== Resources

* {Project page}[http://code.google.com/p/ruby-sequel/]
* {Source code}[http://ruby-sequel.googlecode.com/svn/]
* {Bug tracking}[http://code.google.com/p/ruby-sequel/issues/list]
* {Google group}[http://groups.google.com/group/sequel-talk]
* {RubyForge page}[http://rubyforge.org/projects/sequel/]

To check out the source code:
  
  svn co http://ruby-sequel.googlecode.com/svn/trunk
  
=== Contact

If you have any comments or suggestions please send an email to ciconia at gmail.com and I'll get back to you.

== Installation

  sudo gem install sequel
  
== Supported Databases

Sequel currently supports:

* ADO (on Windows)
* DBI
* Informix
* MySQL
* ODBC
* Oracle
* PostgreSQL
* SQLite 3

There are also experimental adapters for DB2, OpenBase and JDBC (on JRuby).

== The Sequel Console

Sequel includes an IRB console for quick'n'dirty access to databases. You can use it like this:

  sequel sqlite:///test.db

You get an IRB session with the database object stored in DB.

== An Introduction

Sequel was designed to take the hassle away from connecting to databases and manipulating them. Sequel deals with all the boring stuff like maintaining connections, formatting SQL correctly and fetching records so you can concentrate on your application.

Sequel uses the concept of datasets to retrieve data. A Dataset object encapsulates an SQL query and supports chainability, letting you fetch data using a convenient Ruby DSL that is both concise and infinitely flexible.

For example, the following one-liner returns the average GDP for the five biggest countries in the middle east region:

  DB[:countries].filter(:region => 'Middle East').reverse_order(:area).limit(5).avg(:GDP)
  
Which is equivalent to:

  SELECT avg(GDP) FROM countries WHERE region = 'Middle East' ORDER BY area DESC LIMIT 5

Since datasets retrieve records only when needed, they can be stored and later reused. Records are fetched as hashes (they can also be fetched as custom model objects), and are accessed using an Enumerable interface:

  middle_east = DB[:countries].filter(:region => 'Middle East')
  middle_east.order(:name).each {|r| puts r[:name]}
  
Sequel also offers convenience methods for extracting data from Datasets, such as an extended map method:

  middle_east.map(:name) #=> ['Egypt', 'Greece', 'Israel', ...]
  
Or getting results as a transposed hash, with one column as key and another as value:

  middle_east.to_hash(:name, :area) #=> {'Israel' => 20000, 'Greece' => 120000, ...}

Much of Sequel is still undocumented (especially the part relating to model classes). The following section provides examples of common usage. Feel free to explore...

== Getting Started

=== Connecting to a database

To connect to a database you simply provide Sequel with a URL:

  require 'sequel'
  DB = Sequel.open 'sqlite:///blog.db'
  
The connection URL can also include such stuff as the user name and password:

  DB = Sequel.open 'postgres://cico:12345@localhost:5432/mydb'

You can also specify optional parameters, such as the connection pool size, or a logger for logging SQL queries:

  DB = Sequel.open("postgres://postgres:postgres@localhost/my_db",
    :max_connections => 10, :logger => Logger.new('log/db.log'))

=== Arbitrary SQL queries

  DB.execute("create table t (a text, b text)")
  DB.execute("insert into t values ('a', 'b')")

Or more succinctly:

  DB << "create table t (a text, b text)"
  DB << "insert into t values ('a', 'b')"

=== Getting Dataset Instances

Dataset is the primary means through which records are retrieved and manipulated. You can create an blank dataset by using the dataset method:

  dataset = DB.dataset

Or by using the from methods:

  posts = DB.from(:posts)

You can also use the equivalent shorthand:

  posts = DB[:posts]

Note: the dataset will only fetch records when you explicitly ask for them, as will be shown below. Datasets can be manipulated to filter through records, change record order and even join tables, as will also be shown below.

=== Retrieving Records

You can retrieve records by using the all method:

  posts.all

The all method returns an array of hashes, where each hash corresponds to a record.

You can also iterate through records one at a time:

  posts.each {|row| p row}

Or perform more advanced stuff:

  posts.map(:id)
  posts.inject({}) {|h, r| h[r[:id]] = r[:name]}
  
You can also retrieve the first record in a dataset:

  posts.first
  
Or retrieve a single record with a specific value:

  posts[:id => 1]
  
If the dataset is ordered, you can also ask for the last record:

  posts.order(:stamp).last
  
=== Filtering Records

The simplest way to filter records is to provide a hash of values to match:

  my_posts = posts.filter(:category => 'ruby', :author => 'david')
  
You can also specify ranges:

  my_posts = posts.filter(:stamp => (2.weeks.ago)..(1.week.ago))
  
Or lists of values:

  my_posts = posts.filter(:category => ['ruby', 'postgres', 'linux'])
  
Sequel now also accepts expressions as closures, AKA block filters:

  my_posts = posts.filter {:category == ['ruby', 'postgres', 'linux']}
  
Which also lets you do stuff like:

  my_posts = posts.filter {:stamp > 1.month.ago}
  
Some adapters (like postgresql) will also let you specify Regexps:

  my_posts = posts.filter(:category => /ruby/i)
  
You can also use an inverse filter:

  my_posts = posts.exclude(:category => /ruby/i)

You can then retrieve the records by using any of the retrieval methods:

  my_posts.each {|row| p row}
  
You can also specify a custom WHERE clause:

  posts.filter('(stamp < ?) AND (author <> ?)', 3.days.ago, author_name)

Datasets can also be used as subqueries:

  DB[:items].filter('price > ?', DB[:items].select('AVG(price) + 100'))

=== Summarizing Records

Counting records is easy:
  posts.filter(:category => /ruby/i).count

And you can also query maximum/minimum values:
  max_value = DB[:history].max(:value)
  
Or calculate a sum:
  total = DB[:items].sum(:price)
  
=== Ordering Records

  posts.order(:stamp)
  
You can also specify descending order

  posts.order(:stamp.DESC)

=== Deleting Records

  posts.filter('stamp < ?', 3.days.ago).delete
  
=== Inserting Records

  posts.insert(:category => 'ruby', :author => 'david')
  
Or alternatively:

  posts << {:category => 'ruby', :author => 'david'}
  
=== Updating Records

  posts.filter('stamp < ?', 3.days.ago).update(:state => 'archived')

=== Joining Tables

Joining is very useful in a variety of scenarios, for example many-to-many relationships. With Sequel it's really easy:

  order_items = DB[:items].join(:order_items, :item_id => :id).
    filter(:order_items__order_id => 1234)
  
This is equivalent to the SQL:

  SELECT * FROM items LEFT OUTER JOIN order_items
  ON order_items.item_id = items.id 
  WHERE order_items.order_id = 1234

You can then do anything you like with the dataset:

  order_total = order_items.sum(:price)
  
Which is equivalent to the SQL:

  SELECT sum(price) FROM items LEFT OUTER JOIN order_items
  ON order_items.item_id = items.id
  WHERE order_items.order_id = 1234