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The Sequel Object Model

Sequel's dataset layer is mostly structured as an DSL, so it often obscures what actual objects are being used. For example, you don't usually create Sequel objects by calling #new on the object's class (other than Sequel::Model subclasses). However, just as almost everything in ruby is an object, all the methods you call in Sequel deal with objects behind the scenes.

There are five main types of Sequel-specific objects that you deal with in Sequel:

  • Sequel::Database

  • Sequel::Dataset

  • Sequel::Model

  • Standard Ruby Types

  • Sequel::SQL::Expression (and subclasses)


Sequel::Database is the main Sequel object that you deal with. It's usually created by the Sequel.connect method:

DB = Sequel.connect('postgres://host/database')

A Sequel::Database object represents the database you are connecting to. Sequel::Database handles things like Sequel::Dataset creation,

dataset = DB[:table]

schema modification,

DB.create_table(:table) do
  primary_key :id
  String :name

and transactions:

DB.transaction do

Sequel::Database#literal can be used to take any object that Sequel handles and literalize the object to an SQL string fragment:

DB.literal(DB[:table]) # (SELECT * FROM "table")


Sequel::Dataset objects represent SQL queries, or more generally, they represent abstract collections of rows in the database. They are usually created from a Sequel::Database object:

dataset = DB[:table]         # SELECT * FROM "table"
dataset = DB.from(table)     # SELECT * FROM "table"
dataset = # SELECT "column"

Most Sequel::Dataset methods return modified copies of the receiver, and the general way to build queries in Sequel is via a method chain:

dataset = DB[:test].
            select(:column1, :column2).
            where(:column3 => 4).

Such a method chain is a more direct way of doing:

dataset = DB[:test]
dataset =, :column2)
dataset = dataset.where(:column3 => 4)
dataset = dataset.order(:column5)

When you are ready to execute your query, you call one of the Sequel::Dataset action methods. For returning rows, you can do:

dataset.each{|row| row}

For inserting, updating, or deleting rows, you can do:


All datasets are related to their database object, which you can access via the Sequel::Dataset#db method:

dataset.db # => DB


Sequel::Model objects are wrappers around a particular Sequel::Dataset object that add custom behavior, both custom behavior for the entire set of rows in the dataset (the model's class methods), custom behavior for a subset of rows in the dataset (the model's dataset methods), and custom behavior for single rows in the dataset (the model's instance methods).

Unlike most other Sequel objects, Sequel::Model classes and instances are generally created by the user using standard ruby syntax:

class Album < Sequel::Model
album =

All model classes are related to their Sequel::Dataset object, which you can access via the Sequel::Model.dataset method:

Album.dataset # SELECT * FROM "albums"

Additionally, all model classes are related to their dataset's Sequel::Database object, which you can access via the Sequel::Model.db method:

Album.db # => DB

Standard Ruby Types

Where possible, Sequel uses ruby's standard types to represent SQL concepts. In the examples here, the text to the right side of the # sign is the output if you pass the left side to Sequel::Database#literal.


For example, ruby symbols represent SQL identifiers (tables, columns, schemas):

:table  # "table"
:column # "column"

However, they can also represent qualified identifiers by including a double underscore inside a symbol:

:table__column # "table"."column"

They can also represent an aliased identifier by including a triple underscore inside a symbol:

:column___alias # "column" AS "alias"

You can combine both qualification and aliasing by using a double underscore and a triple underscore:

:table__column___alias # "table"."column" AS "alias"

Integer, Float, BigDecimal, String, Date, Time, DateTime

Ruby's Integer, Float, BigDecimal, String, Date, Time, and DateTime classes represent similar types in SQL:

1                     # 1
1.0                   # 1.0'1.0') # 1.0
"string"              # 'string', 5, 6)  # '2012-05-06'              # '2012-05-06 10:20:30'          # '2012-05-06 10:20:30'


Sequel generally uses hash objects to represent equality:

{:column => 1} # ("column" = 1)

However, if you use in array as the hash value, it will usually be used to represent inclusion:

{:column => [1, 2, 3]} # ("column" IN (1, 2, 3))

You can also use a Sequel::Dataset instance as the hash value, which will be used to represent inclusion in the subselect:

{:column => DB[:table].select(:column)} # ("column" IN (SELECT "column" FROM "table"))

If you pass true, false, or nil as the hash value, it will be used to represent identity:

{:column => nil} # ("column" IS NULL)

If you pass a Range object, it will be used as the bounds for a greater than and less than operation:

{:column => 1..2} # (("column" >= 1) AND ("column" <= 2))
{:column => 1...3} # (("column" >= 1) AND ("column" < 3))

If you pass a Regexp object as the value, it will be used as a regular expression operation (only supported on PostgreSQL and MySQL currently):

{:column => /a.*b/} # ("column" ~ 'a.*b')


Sequel generally treats arrays as an SQL value list:

[1, 2, 3] # (1, 2, 3)

However, if all members of the array are arrays with two members, then the array is treated like a hash:

[[:column, 1]] # ("column" = 1)

The advantage of using an array over a hash for such a case is that a hash cannot include multiple objects with the same key, while the array can.

Sequel::SQL::Expression (and subclasses)

If Sequel needs to represent an SQL concept that does not map directly to an existing ruby class, it will generally use a Sequel::SQL::Expression subclass to represent that concept.


Sequel::LiteralString is not actually a Sequel::SQL::Expression subclass. It is a subclass of String, but it is treated specially by Sequel, in that it is treated as literal SQL code, instead of as an SQL string that needs to be escaped:"co'de") # co'de

The following shortcuts exist for creating Sequel::LiteralString objects:



Sequel::SQL::Blob is also a String subclass, but it is treated as an SQL blob instead of an SQL string, as SQL blobs often have different literalization rules than SQL strings do:"blob")

The following shortcuts exist for creating Sequel::SQL::Blob objects:



Sequel::SQLTime is a Time subclass. However, it is treated specially by Sequel in that only the time component is literalized, not the date part. This type is used to represent SQL time types, which do not contain date information.

Sequel::SQLTime.create(10, 20, 30) # "10:20:30"


Sequel::SQL::ValueList objects always represent SQL value lists. Most ruby arrays represent value lists in SQL, except that arrays of two-element arrays are treated similar to hashes. Such arrays can be wrapped in this class to ensure they are treated as value lists. This is important when doing a composite key IN lookup, which some databases support. Sequel::SQL::ValueList is an ::Array subclass with no additional behavior, so it can be instantiated like a normal array:[[1, 2], [3, 4]]) # ((1, 2), (3, 4))

In old versions of Sequel, these objects often needed to be created manually, but in newer versions of Sequel, they are created automatically in most cases where they are required.

The following shortcuts exist for creating Sequel::SQL::ValueList objects:

Sequel.value_list([[1, 2], [3, 4]])
[[1, 2], [3, 4]].sql_value_list


Sequel::SQL::Identifier objects represent single identifiers. The main reason for their existance is that they are not checked for double or triple underscores, so no automatic qualification or aliasing happens for them: # "col__umn"

The following shortcuts exist for creating Sequel::SQL::Identifier objects:



Sequel::SQL::QualifiedIdentifier objects represent qualified identifiers:, :column) # "table"."column"

The following shortcuts exist for creating Sequel::SQL::QualifiedIdentifier objects:

Sequel.qualify(:table, :column)


Sequel::SQL::AliasedExpression objects represent aliased expressions in SQL. The alias is treated as an identifier, but the expression can be an arbitrary Sequel expression:, :alias) # "column" AS "alias"

The following shortcuts exist for creating Sequel::SQL::AliasedExpression objects:, :alias)


Sequel::SQL::ComplexExpression objects mostly represent SQL operations with arguments. There are separate subclasses for representing boolean operations such as AND and OR (Sequel::SQL::BooleanExpression), mathematical operations such as + and - (Sequel::SQL::NumericExpression), and string operations such as || and LIKE (Sequel::SQL::StringExpression)., :col1, :col2) # ("col1" OR "col2"), :column, 2) # ("column" + 2)"||", :column, "b") # ("column" || 'b')

There are many shortcuts for creating Sequel::SQL::ComplexExpression objects:

Sequel.or(:col1, :col2)
:col1 | :col2

Sequel.+(:column, 2)
:column + 2

Sequel.join([:column, 'b'])
:column + 'b'


Sequel::SQL::CaseExpression objects represent SQL CASE expressions, which represent branches in the database, similar to ruby case expressions. Like ruby's case expressions, these case expressions can have a implicit value you are comparing against:{2=>1}, 0, :a) # CASE "a" WHEN 2 THEN 1 ELSE 0 END

Or they can treat each condition separately:{{:a=>2}=>1}, 0) # CASE WHEN ("a" = 2) THEN 1 ELSE 0 END

In addition to providing a hash, you can also provide an array of two-element arrays:[[2, 1]], 0, :a) # CASE "a" WHEN 2 THEN 1 ELSE 0 END

The following shortcuts exist for creating Sequel::SQL::CaseExpression objects:{2=>1}, 0, :a){{:a=>2}=>1}, 0)

{2=>1}.case(0, :a)


Sequel::SQL::Cast objects represent CAST expressions in SQL, which does explicit typecasting in the database. With Sequel, you provide the expression to typecast as well as the type to cast to. The type can either be a generic type, given as a ruby class:, String) # (CAST "a" AS text)

or a specific type, given as a symbol or string:, :int4) # (CAST "a" AS int4)

The following shortcuts exist for creating Sequel::SQL::Cast objects:

Sequel.cast(:a, String)
Sequel.cast(:a, :int4)



Sequel::SQL::ColumnAll objects represent the selection of all columns from a table. They are pretty much only used as arguments to one of the Dataset select methods, and are not used much anymore since Dataset#select_all was expanded to take arguments. Still, it's possible they are still useful in some code: # "table".*

The following shortcut exists for creating Sequel::SQL::Cast objects:



Sequel::SQL::Constant objects represent constants or psuedo-constants in SQL, such as TRUE, NULL, and CURRENT_TIMESTAMP. These are not designed to be created or used by the end user, but some existing values are predefined under the Sequel namespace:


These objects are usually used as values in queries:



Sequel::SQL::Function objects represents database function calls, which take a function name and any arguments:, :a, 2) # func("a", 2)

The following shortcuts exist for creating Sequel::SQL::Function objects:

Sequel.function(:func, :a, 2)
:func.sql_function(:a, 2)


Sequel::SQL::JoinClause objects represent SQL JOIN clauses. They are usually not created manually, as the Dataset join methods create them automatically.


Sequel::SQL::PlaceholderLiteralString objects represent a literal SQL string with placeholders for variables. There are three types of these objects. The first type uses question marks with multiple placeholder value objects:'? = ?', [:a, 1]) # "a" = 1

The second uses named placeholders with colons and a hash of placeholder value objects:':b = :v', [{:b=>:a, :v=>1}]) # "a" = 1

The third uses an array instead of a string, with multiple placeholder objects, each one going in between the members of the array:['', ' = '], [:a, 1]) # "a" = 1

For any of these three forms, you can also include a third argument for whether to include parentheses around the string:'? = ?', [:a, 1], true) # ("a" = 1)

The following shortcuts exist for creating Sequel::SQL::PlaceholderLiteralString objects:

Sequel.lit('? = ?', :a, 1)
Sequel.lit(':b = :v', :b=>:a, :v=>1)
Sequel.lit(['', ' = '], :a, 1)

'? = ?'.lit(:a, 1)
':b = :v'.lit(:b=>:a, :v=>1)


Sequel::SQL::OrderedExpression objects represent ascending or descending sorts, used by the Dataset order methods. They take an expression, and whether to sort it ascending or descending: # "a" DESC, false) # "a" ASC

Additionally, they take an options hash, which can be used to specify how nulls can be sorted:, true, :nulls=>:first) # "a" DESC NULLS FIRST, false, :nulls=>:last) # "a" ASC NULLS LAST

The following shortcuts exist for creating Sequel::SQL::OrderedExpression objects:

Sequel.asc(:a, :nulls=>:first)
Sequel.desc(:a, :nulls=>:last)



Sequel::SQL::Subscript objects represent SQL database array access. They take an expression and an array of indexes:, [1]) # "a"[1], [1, 2]) # "a"[1, 2]

The following shortcuts exist for creating Sequel::SQL::Subscript objects:

Sequel.subscript(:a, 1)
Sequel.subscript(:a, 1, 2)

:a.sql_subscript(1, 2)


Sequel::SQL::VirtualRow is a BasicObject subclass that is the backbone behind the block expression support:

DB[:table].filter{a < 1}

In the above code, the block is instance-evaled instead a VirtualRow instance.

These objects are usually not instantiated manually. See the Virtual Row Guide for details.


Sequel::SQL::Window objects represent the windows used by Sequel::SQL::WindowFunction. They use a hash-based API, supporting the :frame, :order, :partition, and :window options:>:a) # (ORDER BY "a")>:a) # (PARTITION BY "a")>:a, :frame=>:all)


Sequel::SQL::WindowFunction objects represent SQL window function calls. These just combine a Sequel::SQL::Function with a Sequel::SQL::Window:

function =, 1)
window =>:a), window) # f(1) OVER (ORDER BY "a")

Virtual rows offer a shortcut for creating Sequel::SQL::Window objects.


Sequel::SQL::Wrapper objects wrap arbitrary objects so that they can be used in Sequel expressions:

o =
def o.sql_literal(ds) "foo" end # foo

The advantage of wrapping the object is that you can the call Sequel methods on the wrapper that would not be defined on the object itself: + 1 # (foo + 1)

You can use the Sequel.expr method to wrap any object:


However, note that that does not necessarily return a Sequel::SQL::Wrapper object, it may return a different class of object, such as a Sequel::SQL::ComplexExpression subclass object.

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