Sequel: The Database Toolkit for Ruby
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Sequel: The Database Toolkit for Ruby

Sequel is a lightweight database access toolkit for Ruby.

  • Sequel provides thread safety, connection pooling and a concise DSL for constructing database queries and table schemas.

  • Sequel also includes a lightweight but comprehensive ORM layer for mapping records to Ruby objects and handling associated records.

  • Sequel supports advanced database features such as prepared statements, bound variables, master/slave configurations, and database sharding.

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

  • Sequel currently has adapters for ADO, DB2, DBI, Informix, JDBC, MySQL, ODBC, OpenBase, Oracle, PostgreSQL and SQLite3.


To check out the source code:

git clone git://


If you have any comments or suggestions please post to the Google group.


sudo gem install sequel

A Short Example

require 'rubygems'
require 'sequel'

DB = Sequel.sqlite # memory database

DB.create_table :items do # Create a new table
  primary_key :id
  column :name, :text
  column :price, :float

items = DB[:items] # Create a dataset

# Populate the table
items << {:name => 'abc', :price => rand * 100}
items << {:name => 'def', :price => rand * 100}
items << {:name => 'ghi', :price => rand * 100}

# Print out the number of records
puts "Item count: #{items.count}"

# Print out the records in descending order by price

# Print out the average price
puts "The average price is: #{items.avg(:price)}"

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 # test.db in current directory

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

An Introduction

Sequel is 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: #=> ['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, ...}

Getting Started

Connecting to a database

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

require 'sequel'
DB = Sequel.connect('sqlite://blog.db')

The connection URL can also include such stuff as the user name and password:

DB = Sequel.connect('postgres://cico:12345@localhost:5432/mydb')

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

DB = Sequel.connect("postgres://postgres:postgres@localhost/my_db",
  :max_connections => 10, :loggers => ['log/db.log']))

You can specify a block to connect, which will disconnect from the database after it completes:

Sequel.connect('postgres://cico:12345@localhost:5432/mydb'){|db| db[:posts].delete}

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')"

You can also create datasets based on raw SQL:

dataset = DB['select * from items']
dataset.count # will return the number of records in the result set # will return an array containing all values of the id column in the result set

You can also fetch records with raw SQL through the dataset:

DB['select * from items'].each do |row|
  p row

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)

The recommended way is the equivalent shorthand:

posts = DB[:posts]

Datasets will only fetch records when you explicitly ask for them. Datasets can be manipulated to filter through records, change record order, join tables, etc..

Retrieving Records

You can retrieve records by using the all method:


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.inject({}){|h, r| h[r[:id]] = r[:name]}

You can also retrieve the first record in a dataset:


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:


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 => ( - 14)..( - 7))

Or lists of values:

my_posts = posts.filter(:category => ['ruby', 'postgres', 'linux'])

Sequel also accepts expressions:

my_posts = posts.filter(:stamp > << 1)

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)
my_posts = posts.filter(:category => /ruby/i).invert # same as above

You can also specify a custom WHERE clause using a string:

posts.filter('stamp IS NOT NULL')

You can use parameters in your string, as well (ActiveRecord style):

posts.filter('(stamp < ?) AND (author != ?)', - 3, author_name)
posts.filter((:stamp < - 3) & ~(:author => author_name)) # same as above

Datasets can also be used as subqueries:

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

After filtering you can retrieve the matching records by using any of the retrieval methods:

my_posts.each{|row| p row}

See the doc/dataset_filtering.rdoc file for more details.

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

Ordering datasets is simple:

posts.order(:stamp) # ORDER BY stamp
posts.order(:stamp, :name) # ORDER BY stamp, name

You can also specify descending order

posts.order(:stamp.desc) # ORDER BY stamp DESC

Deleting Records

Deleting records from the table is done with delete:

posts.filter('stamp < ?', - 3).delete

Inserting Records

Inserting records into the table is done with insert:

posts.insert(:category => 'ruby', :author => 'david')
posts << {:category => 'ruby', :author => 'david'} # same as above

Updating Records

Updating records in the table is done with update:

posts.filter('stamp < ?', - 7).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 = 
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 =
WHERE order_items.order_id = 1234

Graphing Datasets

When retrieving records from joined datasets, you get the results in a single hash, which is subject to clobbering:

DB[:items].join(:order_items, :item_id => :id).first
=> {:id=>(could be or, :item_id=>order_items.order_id}

Using graph, you can split the result hashes into subhashes, one per join:

DB[:items].graph(:order_items, :item_id => :id).first
=> {:items=>{:id=>}, :order_items=>{:id=>, :item_id=>order_items.item_id}}

Sequel Models

Models in Sequel are based on the Active Record pattern described by Martin Fowler ( A model class corresponds to a table or a dataset, and an instance of that class wraps a single record in the model's underlying dataset.

Model classes are defined as regular Ruby classes:

DB = Sequel.connect('sqlite:/blog.db')
class Post < Sequel::Model

Just like in DataMapper or ActiveRecord, Sequel model classes assume that the table name is a plural of the class name:

Post.table_name #=> :posts

You can, however, explicitly set the table name or even the dataset used:

class Post < Sequel::Model(:my_posts)
# or:
Post.set_dataset :my_posts
# or:
Post.set_dataset DB[:my_posts].where(:category => 'ruby')

Model instances

Model instance are identified by a primary key. By default, Sequel assumes the primary key column to be :id. The Model#[] method can be used to fetch records by their primary key:

post = Post[123]

The Model#pk method is used to retrieve the record's primary key value: #=> 123

Sequel models allow you to use any column as a primary key, and even composite keys made from multiple columns:

class Post < Sequel::Model
  set_primary_key [:category, :title]

post = Post['ruby', 'hello world'] #=> ['ruby', 'hello world']

You can also define a model class that does not have a primary key, but then you lose the ability to update records.

A model instance can also be fetched by specifying a condition:

post = Post[:title => 'hello world']
post = Post.find(:num_comments < 10)

Iterating over records

A model class lets you iterate over specific records by acting as a proxy to the underlying dataset. This means that you can use the entire Dataset API to create customized queries that return model instances, e.g.:

Post.filter(:category => 'ruby').each{|post| p post}

You can also manipulate the records in the dataset:

Post.filter(:num_comments < 7).delete
Post.filter( => 'ruby')

Accessing record values

A model instances stores its values as a hash:

post.values #=> {:id => 123, :category => 'ruby', :title => 'hello world'}

You can read the record values as object attributes (assuming the attribute names are valid columns in the model's dataset): #=> 123
post.title #=> 'hello world'

You can also change record values:

post.title = 'hey there'

Another way to change values by using the #update_with_params method:

post.update_with_params(:title => 'hey there')

Creating new records

New records can be created by calling Model.create:

post = Post.create(:title => 'hello world')

Another way is to construct a new instance and save it:

post =
post.title = 'hello world'

You can also supply a block to and Model.create:

post = Post.create {|p| p.title = 'hello world'}

post = do |p|
  p.title = 'hello world'


You can execute custom code when creating, updating, or deleting records by using hooks. The before_create and after_create hooks wrap record creation. The before_update and after_update wrap record updating. The before_save and after_save wrap record creation and updating. The before_destroy and after_destroy wrap destruction. The before_validation and after_validation hooks wrap validation.

Hooks are defined by supplying a block:

class Post < Sequel::Model
  after_create do

  after_destroy do

Deleting records

You can delete individual records by calling #delete or #destroy. The only difference between the two methods is that #destroy invokes before_destroy and after_destroy hooks, while #delete does not:

post.delete #=> bypasses hooks
post.destroy #=> runs hooks

Records can also be deleted en-masse by invoking Model.delete and Model.destroy. As stated above, you can specify filters for the deleted records:

Post.filter(:category => 32).delete #=> bypasses hooks
Post.filter(:category => 32).destroy #=> runs hooks

Please note that if Model.destroy is called, each record is deleted separately, but Model.delete deletes all relevant records with a single SQL statement.


Associations are used in order to specify relationships between model classes that reflect relations between tables in the database using foreign keys.

class Post < Sequel::Model
  many_to_one :author
  one_to_many :comments
  many_to_many :tags

You can also use the ActiveRecord names for these associations:

class Post < Sequel::Model
  belongs_to :author
  has_many :comments
  has_and_belongs_to_many :tags

many_to_one creates a getter and setter for each model object:

class Post < Sequel::Model
  many_to_one :author

post = Post.create(:name => 'hi!') = Author[:name => 'Sharon']

one_to_many and many_to_many create a getter method, a method for adding an object to the association, a method for removing an object from the association, and a method for removing all associated objected from the association:

class Post < Sequel::Model
  one_to_many :comments
  many_to_many :tags

post = Post.create(:name => 'hi!')
comment = Comment.create(:text=>'hi')
tag = Tag.create(:tag=>'interesting')

Eager Loading

Associations can be eagerly loaded via .eager and the :eager association option. Eager loading is used when loading a group of objects. It loads all associated objects for all of the current objects in one query, instead of using a separate query to get the associated objects for each current object. Eager loading requires that you retrieve all model objects at once via .all (instead of individually by .each). Eager loading can be cascaded, loading association's associated objects.

class Person < Sequel::Model
  one_to_many :posts, :eager=>[:tags]

class Post < Sequel::Model
  many_to_one :person
  one_to_many :replies
  many_to_many :tags

class Tag < Sequel::Model
  many_to_many :posts
  many_to_many :replies

class Reply < Sequel::Model
  many_to_one :person
  many_to_one :post
  many_to_many :tags

# Eager loading via .eager

# eager is a dataset method, so it works with filters/orders/limits/etc.
Post.filter(:topic > 'M').order(:date).limit(5).eager(:person).all

person = Person.first
# Eager loading via :eager (will eagerly load the tags for this person's posts)

# These are equivalent
Post.eager(:person, :tags).all

# Cascading via .eager

# Will also grab all associated posts' tags (because of :eager)

# No depth limit (other than memory/stack), and will also grab posts' tags
# Loads all people, their posts, their posts' tags, replies to those posts,
# the person for each reply, the tag for each reply, and all posts and
# replies that have that tag.  Uses a total of 8 queries.
Person.eager(:posts=>{:replies=>[:person, {:tags=>{:posts, :replies}}]}).all

In addition to using eager, you can also use eager_graph, which will use a single query to get the object and all associated objects. This may be necessary if you want to filter the result set based on columns in associated tables. It works with cascading as well, the syntax is exactly the same. Note that using eager_graph to eagerly load multiple *_to_many associations will cause the result set to be a cartesian product, so you should be very careful with your filters when using it in that case.

Caching model instances with memcached

Sequel models can be cached using memcached based on their primary keys. The use of memcached can significantly reduce database load by keeping model instances in memory. The set_cache method is used to specify caching:

require 'memcache'
CACHE = 'localhost:11211', :namespace => 'blog'

class Author < Sequel::Model
  set_cache CACHE, :ttl => 3600

Author[333] # database hit
Author[333] # cache hit

Extending the underlying dataset

The obvious way to add table-wide logic is to define class methods to the model class definition. That way you can define subsets of the underlying dataset, change the ordering, or perform actions on multiple records:

class Post < Sequel::Model
  def self.posts_with_few_comments
    filter(:num_comments < 30)

  def self.clean_posts_with_few_comments

You can also implement table-wide logic by defining methods on the dataset:

class Post < Sequel::Model
  def_dataset_method(:posts_with_few_comments) do
    filter(:num_comments < 30)

  def_dataset_method(:clean_posts_with_few_comments) do

This is the recommended way of implementing table-wide operations, and allows you to have access to your model API from filtered datasets as well:

Post.filter(:category => 'ruby').clean_old_posts

Sequel models also provide a short hand notation for filters:

class Post < Sequel::Model
  subset(:posts_with_few_comments, :num_comments < 30)
  subset :invisible, :visible => false

Defining the underlying schema

Model classes can also be used as a place to define your table schema and control it. The schema DSL is exactly the same provided by Sequel::Schema::Generator:

class Post < Sequel::Model
  set_schema do
    primary_key :id
    text :title
    text :category
    foreign_key :author_id, :table => :authors

You can then create the underlying table, drop it, or recreate it:

Post.create_table! # drops the table if it exists and then recreates it

Basic Model Validations

To assign default validations to a sequel model:

class MyModel < Sequel::Model
  validates do

You may also perform the usual 'longhand' way to assign default model validates directly within the model class itself:

class MyModel < Sequel::Model