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Sequel: The Database Toolkit for Ruby

Sequel is a simple, flexible, and powerful SQL database access toolkit for Ruby.

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

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

  • Sequel supports advanced database features such as prepared statements, bound variables, stored procedures, savepoints, two-phase commit, transaction isolation, master/slave configurations, and database sharding.

  • Sequel currently has adapters for ADO, Amalgalite, CUBRID, DataObjects, DB2, DBI, Firebird, IBM_DB, Informix, JDBC, MySQL, Mysql2, ODBC, OpenBase, Oracle, PostgreSQL, SQLAnywhere, SQLite3, Swift, and TinyTDS.


To check out the source code:

git clone git://


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


gem install sequel

A Short Example

require 'sequel'

DB = Sequel.sqlite # memory database

DB.create_table :items do
  primary_key :id
  String :name
  Float :price

items = DB[:items] # Create a dataset

# Populate the table
items.insert(:name => 'abc', :price => rand * 100)
items.insert(:name => 'def', :price => rand * 100)
items.insert(:name => 'ghi', :price => rand * 100)

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

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

The Sequel Console

Sequel includes an IRB console for quick access to databases (usually referred to as bin/sequel). 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.

In addition to providing an IRB shell (the default behavior), bin/sequel also has support for migrating databases, dumping schema migrations, and copying databases. See the bin/sequel guide for more details.

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 flexible.

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

DB[:countries].filter(:region => 'Middle East').avg(:GDP)

Which is equivalent to:

SELECT avg(GDP) FROM countries WHERE region = 'Middle East'

Since datasets retrieve records only when needed, they can be stored and later reused. Records are fetched as hashes (or 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', 'Turkey', 'Israel', ...]

Or getting results as a hash via to_hash, with one column as key and another as value:

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

Getting Started

Connecting to a database

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

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

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

DB = Sequel.connect('postgres://user:password@host:port/database_name')

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

DB = Sequel.connect("postgres://user:password@host:port/database_name",
  :max_connections => 10, :logger =>'log/db.log'))

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

Sequel.connect('postgres://user:password@host:port/database_name'){|db| db[:posts].delete}

The DB convention

Throughout Sequel's documentation, you will see the DB constant used to refer to the Sequel::Database instance you create. This reflects the recommendation that for an app with a single Sequel::Database instance, the Sequel convention is to store the instance in the DB constant. This is just a convention, it's not required, but it is recommended.

Note that some frameworks that use Sequel may create the Sequel::Database instance for you, and you might not know how to access it. In most cases, you can access the Sequel::Database instance through Sequel::Model.db.

Arbitrary SQL queries

You can execute arbitrary SQL code using Database#run:"create table t (a text, b text)")"insert into t values ('a', 'b')")

You can also create datasets based on raw SQL:

dataset = DB['select id 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

You can use placeholders in your SQL string as well:

name = 'Jim'
DB['select * from items where name = ?', name].each do |row|
  p row

Getting Dataset Instances

Datasets are the primary way records are retrieved and manipulated. They are generally created via the Database#from or Database#[] methods:

posts = DB.from(:posts)
posts = DB[:posts] # same

Datasets will only fetch records when you tell them to. They can be manipulated to filter records, change ordering, join tables, etc..

Retrieving Records

You can retrieve all records by using the all method:

# SELECT * FROM posts

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 using each:

posts.each{|row| p row}

Or perform more advanced stuff:

names_and_dates =[:name, :date])
old_posts, recent_posts = posts.partition{|r| r[:date] < - 7}

You can also retrieve the first record in a dataset:


Or retrieve a single record with a specific value:

posts[:id => 1]
# SELECT * FROM posts WHERE id = 1 LIMIT 1

If the dataset is ordered, you can also ask for the last record:


Filtering Records

An easy way to filter records is to provide a hash of values to match to where:

my_posts = posts.where(:category => 'ruby', :author => 'david')
# WHERE category = 'ruby' AND author = 'david'

You can also specify ranges:

my_posts = posts.where(:stamp => ( - 14)..( - 7))
# WHERE stamp >= '2010-06-30' AND stamp <= '2010-07-07'

Or arrays of values:

my_posts = posts.where(:category => ['ruby', 'postgres', 'linux'])
# WHERE category IN ('ruby', 'postgres', 'linux')

Sequel also accepts expressions:

my_posts = posts.where{stamp > << 1}
# WHERE stamp > '2010-06-14'

Some adapters will also let you specify Regexps:

my_posts = posts.where(:category => /ruby/i)
# WHERE category ~* 'ruby'

You can also use an inverse filter via exclude:

my_posts = posts.exclude(:category => ['ruby', 'postgres', 'linux'])
# WHERE category NOT IN ('ruby', 'postgres', 'linux')

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

posts.where('stamp IS NOT NULL')

You can use parameters in your string, as well:

author_name = 'JKR'
posts.where('(stamp < ?) AND (author != ?)', - 3, author_name)
# WHERE (stamp < '2010-07-11') AND (author != 'JKR')

Datasets can also be used as subqueries:

DB[:items].where('price > ?', DB[:items].select{avg(price) + 100})
# WHERE price > (SELECT avg(price) + 100 FROM items)

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

my_posts.each{|row| p row}

See the Dataset Filtering file for more details.


Designing apps with security in mind is a best practice. Please read the Security Guide for details on security issues that you should be aware of when using Sequel.

Summarizing Records

Counting records is easy using count:

# SELECT COUNT(*) FROM posts WHERE category LIKE '%ruby%'

And you can also query maximum/minimum values via max and min:

max = DB[:history].max(:value)
# SELECT max(value) FROM history

min = DB[:history].min(:value)
# SELECT min(value) FROM history

Or calculate a sum or average via sum and avg:

sum = DB[:items].sum(:price)
# SELECT sum(price) FROM items
avg = DB[:items].avg(:price)
# SELECT avg(price) FROM items

Ordering Records

Ordering datasets is simple using order:

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

Chaining order doesn't work the same as where:

# ORDER BY name

The order_append method chains this way, though:

# ORDER BY stamp, name

The order_prepend method can be used as well:

# ORDER BY name, stamp

You can also specify descending order:


Core Extensions

Note the use of Sequel.desc(:stamp) in the above example. Much of Sequel's DSL uses this style, calling methods on the Sequel module that return SQL expression objects. Sequel also ships with a core_extensions extension) that integrates Sequel's DSL better into the ruby language, allowing you to write:


instead of:


Selecting Columns

Selecting specific columns to be returned is also simple using select:
# SELECT stamp FROM posts, :name)
# SELECT stamp, name FROM posts

Chaining select works like order, not where:
# SELECT name FROM posts

As you might expect, there is an order_append equivalent for select called select_append:
# SELECT stamp, name FROM posts

Deleting Records

Deleting records from the table is done with delete:

posts.where('stamp < ?', - 3).delete
# DELETE FROM posts WHERE stamp < '2010-07-11'

Be very careful when deleting, as delete affects all rows in the dataset. Call where first and delete second:

posts.where('stamp < ?', - 7).delete
posts.delete.where('stamp < ?', - 7)

Inserting Records

Inserting records into the table is done with insert:

posts.insert(:category => 'ruby', :author => 'david')
# INSERT INTO posts (category, author) VALUES ('ruby', 'david')

Updating Records

Updating records in the table is done with update:

posts.where('stamp < ?', - 7).update(:state => 'archived')
# UPDATE posts SET state = 'archived' WHERE stamp < '2010-07-07'

You can reference table columns when choosing what values to set:

posts.where{|o| o.stamp < - 7}.update(:backup_number => Sequel.+(:backup_number, 1))
# UPDATE posts SET backup_number = backup_number + 1 WHERE stamp < '2010-07-07'

As with delete, update affects all rows in the dataset, so where first, update second:

posts.where('stamp < ?', - 7).update(:state => 'archived')
posts.update(:state => 'archived').where('stamp < ?', - 7)


You can wrap some code in a database transaction using the Database#transaction method:

DB.transaction do
  posts.insert(:category => 'ruby', :author => 'david')
  posts.where('stamp < ?', - 7).update(:state => 'archived')

If the block does not raise an exception, the transaction will be committed. If the block does raise an exception, the transaction will be rolled back, and the exception will be reraised. If you want to rollback the transaction and not raise an exception outside the block, you can raise the Sequel::Rollback exception inside the block:

DB.transaction do
  posts.insert(:category => 'ruby', :author => 'david')
  if posts.filter('stamp < ?', - 7).update(:state => 'archived') == 0
    raise Sequel::Rollback

Joining Tables

Sequel makes it easy to join tables:

order_items = DB[:items].join(:order_items, :item_id => :id).
  where(:order_id => 1234)
# SELECT * FROM items INNER JOIN order_items
# ON order_items.item_id = 
# WHERE order_id = 1234

The important thing to note here is that item_id is automatically qualified with the table being joined, and id is automatically qualified with the last table joined.

You can then do anything you like with the dataset:

order_total = order_items.sum(:price)
# SELECT sum(price) FROM items INNER JOIN order_items
# ON order_items.item_id =
# WHERE order_items.order_id = 1234

Column references in Sequel

Sequel expects column names to be specified using symbols. In addition, returned hashes always use symbols as their keys. This allows you to freely mix literal values and column references in many cases. For example, the two following lines produce equivalent SQL:

items.where(:x => 1)
# SELECT * FROM items WHERE (x = 1)
items.where(1 => :x)
# SELECT * FROM items WHERE (1 = x)"

Ruby strings are generally treated as SQL strings:

items.where(:x => 'x')
# SELECT * FROM items WHERE (x = 'x')

Qualifying identifiers (column/table names)

An identifier in SQL is a name that represents a column, table, or schema. Identifiers can be qualified by using the double underscore special notation :table__column:

# items.price

Another way to qualify columns is to use the Sequel.qualify method:

items.literal(Sequel.qualify(:items, :price))
# items.price

While it is more common to qualify column identifiers with table identifiers, you can also qualify table identifiers with schema identifiers to select from a qualified table:

posts = DB[:some_schema__posts]
# SELECT * FROM some_schema.posts

Identifier aliases

You can also alias identifiers by using the triple undersecore special notation :column___alias or :table__column___alias:

# price AS p
# items.price AS p

Another way to alias columns is to use the method:

items.literal(, :p))
# price AS p

You can use the method to alias arbitrary expressions, not just identifiers:

items.literal([:posts].select{max(id)}, :p))
# (SELECT max(id) FROM posts) AS p

Sequel Models

A model class wraps a dataset, and an instance of that class wraps a single record in the dataset.

Model classes are defined as regular Ruby classes inheriting from Sequel::Model:

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

When a model class is created, it parses the schema in the table from the database, and automatically sets up accessor methods for all of the columns in the table (Sequel::Model implements the active record pattern).

Sequel model classes assume that the table name is an underscored plural of the class name:

Post.table_name #=> :posts

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

class Post < Sequel::Model(:my_posts)
# or:
Post.set_dataset :my_posts

If you call set_dataset with a symbol, it assumes you are referring to the table with the same name. You can also call it with a dataset, which will set the defaults for all retrievals for that model:

Post.set_dataset DB[:my_posts].where(:category => 'ruby')
Post.set_dataset DB[:my_posts].select(:id, :name).order(:date)

Model instances

Model instances are identified by a primary key. In most cases, Sequel can query the database to determine the primary key, but if not, it defaults to using :id. The Model.[] method can be used to fetch records by their primary key:

post = Post[123]

The 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 via no_primary_key, but then you lose the ability to easily update and delete records:


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

post = Post[:title => 'hello world']
post = Post.first{num_comments < 10}

Acts like a dataset

A model class forwards many methods to the underlying dataset. This means that you can use most of the Dataset API to create customized queries that return model instances, e.g.:

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

You can also manipulate the records in the dataset:

Post.where{num_comments < 7}.delete
Post.where(, /ruby/)).update(:category => 'ruby')

Accessing record values

A model instance stores its values as a hash with column symbol keys, which you can access directly via the values method:

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'

If the record's attributes names are not valid columns in the model's dataset (maybe because you used select_append to add a computed value column), you can use Model#[] to access the values:

post[:id] #=> 123
post[:title] #=> 'hello world'

You can also modify record values using attribute setters or the []= method.

post.title = 'hey there'
post[:title] = 'hey there'

That will just change the value for the object, it will not update the row in the database. To update the database row, call the save method:

Mass assignment

You can also set the values for multiple columns in a single method call, using one of the mass-assignment methods. See the mass assignment guide for details. For example set updates the model's column values without saving:

post.set(:title=>'hey there', :updated_by=>'foo')

and update updates the model's column values and then saves the changes to the database:

post.update(:title => 'hey there', :updated_by=>'foo')

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 later:

post =
post.title = 'hello world'

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

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

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


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

class Post < Sequel::Model
  def after_create

  def after_destroy

Note the use of super if you define your own hook methods. Almost all Sequel::Model class and instance methods (not just hook methods) can be overridden safely, but you have to make sure to call super when doing so, otherwise you risk breaking things.

For the example above, you should probably use a database trigger if you can. Hooks can be used for data integrity, but they will only enforce that integrity when you are modifying the database through model instances, and even then they are often subject to race conditions. It's best to use database triggers and constraints to enforce data integrity.

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 hook methods, while delete does not:

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

Records can also be deleted en-masse by calling delete and destroy on the model's dataset. As stated above, you can specify filters for the deleted records:

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

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


Associations are used in order to specify relationships between model classes that reflect relationships between tables in the database, which are usually specified using foreign keys. You specify model associations via class methods:

class Post < Sequel::Model
  many_to_one :author
  one_to_many :comments
  one_to_one :first_comment, :class=>:Comment, :order=>:id
  many_to_many :tags
  one_through_one :first_tag, :class=>:Tag, :order=>:name, :right_key=>:tag_id

many_to_one and one_to_one create a getter and setter for each model object:

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 objects from the association:

post = Post.create(:name => 'hi!')

comment = Comment.create(:text=>'hi')

tag = Tag.create(:tag=>'interesting')

Note that the remove_* and remove_all_* methods do not delete the object from the database, they merely disassociate the associated object from the receiver.

All associations add a dataset method that can be used to further filter or reorder the returned objects, or modify all of them:

# Delete all of this post's comments from the database

# Return all tags related to this post with no subscribers, ordered by the tag's name

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.where{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 or order the result set based on columns in associated tables. It works with cascading as well, the API is very similar. 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.

You can dynamically customize the eagerly loaded dataset by using using a proc. This proc is passed the dataset used for eager loading, and should return a modified copy of that dataset:

# Eagerly load only replies containing 'foo'
Post.eager(:replies=>proc{|ds| ds.where(, '%foo%'))}).all

This also works when using eager_graph, in which case the proc is called with dataset to graph into the current dataset:

Post.eager_graph(:replies=>proc{|ds| ds.where(, '%foo%'))}).all

You can dynamically customize eager loads for both eager and eager_graph while also cascading, by making the value a single entry hash with the proc as a key, and the cascaded associations as the value:

# Eagerly load only replies containing 'foo', and the person and tags for those replies
Post.eager(:replies=>{proc{|ds| ds.where(, '%foo%'))}=>[:person, :tags]}).all

Joining with Associations

You can use the association_join method to add a join to the model's dataset based on the assocation:

# SELECT * FROM posts
# INNER JOIN authors AS author ON ( = posts.author_id)

This comes with variants for different join types:

# SELECT * FROM posts
# LEFT JOIN replies ON (replies.post_id =

Similar to the eager loading methods, you can use multiple associations and nested associations:

Post.association_join(:author, :replies=>:person).all
# SELECT * FROM posts
# INNER JOIN authors AS author ON ( = posts.author_id)
# INNER JOIN replies ON (replies.post_id =
# INNER JOIN people AS person ON ( = replies.person_id)

Extending the underlying dataset

The recommended way to implement table-wide logic by defining methods on the dataset using dataset_module:

class Post < Sequel::Model
  dataset_module do
    def posts_with_few_comments
      where{num_comments < 30}

    def clean_posts_with_few_comments

This allows you to have access to your model API from filtered datasets as well:

Post.where(:category => 'ruby').clean_posts_with_few_comments

Sequel models also provide a subset class method that creates a dataset method with a simple filter:

class Post < Sequel::Model
  subset(:posts_with_few_comments){num_comments < 30}
  subset :invisible, Sequel.~(:visible)

Model Validations

You can define a validate method for your model, which save will check before attempting to save the model in the database. If an attribute of the model isn't valid, you should add a error message for that attribute to the model object's errors. If an object has any errors added by the validate method, save will raise an error or return false depending on how it is configured (the raise_on_save_failure flag).

class Post < Sequel::Model
  def validate
    errors.add(:name, "can't be empty") if name.empty?
    errors.add(:written_on, "should be in the past") if written_on >=
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