== 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, savepoints, two-phase commit,
  transaction isolation, primary/replica configurations, and
  database sharding.
* Sequel currently has adapters for ADO, Amalgalite, 
  IBM_DB, JDBC, MySQL, Mysql2, ODBC, Oracle,
  PostgreSQL, SQLAnywhere, SQLite3, TinyTDS, and Trilogy.

== Resources

Website :: https://sequel.jeremyevans.net
RDoc Documentation :: https://sequel.jeremyevans.net/rdoc
Source Code :: https://github.com/jeremyevans/sequel
Bug tracking (GitHub Issues) :: https://github.com/jeremyevans/sequel/issues
Discussion Forum (GitHub Discussions) :: https://github.com/jeremyevans/sequel/discussions
Alternate Discussion Forum (sequel-talk Google Group) :: http://groups.google.com/group/sequel-talk

If you have questions about how to use Sequel, please ask on
GitHub Discussions or the sequel-talk Google Group.
Only use the the bug tracker to report
bugs in Sequel, not to ask for help on using Sequel.

To check out the source code:
  
  git clone git://github.com/jeremyevans/sequel.git
  
=== Contact

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

== Installation

  gem install sequel
  
== A Short Example

  require 'sequel'
  
  DB = Sequel.sqlite # memory database, requires sqlite3
  
  DB.create_table :items do
    primary_key :id
    String :name
    Float :price
  end
  
  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 <tt>bin/sequel</tt>). You can use it like this:

  sequel sqlite://test.db # test.db in current directory

You get an IRB session with the Sequel::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}[rdoc-ref:doc/bin_sequel.rdoc] 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].where(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, and are accessed using an +Enumerable+ interface:

  middle_east = DB[:countries].where(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', 'Turkey', 'Israel', ...]
  middle_east.map([:id, :name]) # => [[1, 'Egypt'], [3, 'Turkey'], [2, 'Israel'], ...]
  
Or getting results as a hash via +as_hash+, with one column as key and another as value:

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

== Getting Started

=== Connecting to a database

To connect to a database you simply provide <tt>Sequel.connect</tt> with a URL:

  require 'sequel'
  DB = Sequel.connect('sqlite://blog.db') # requires sqlite3
  
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') # requires pg

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: Logger.new('log/db.log'))

It is also possible to use a hash instead of a connection URL, but make sure to include the :adapter option in this case:

  DB = Sequel.connect(adapter: :postgres, user: 'user', password: 'password', host: 'host', port: port,
    database: 'database_name', max_connections: 10, logger: Logger.new('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 <tt>Sequel::Model.db</tt>.

=== Arbitrary SQL queries

You can execute arbitrary SQL code using <tt>Database#run</tt>:

  DB.run("create table t (a text, b text)")
  DB.run("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
  dataset.map(:id) # 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
  end

You can use placeholders in your SQL string as well:

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

=== Getting Dataset Instances

Datasets are the primary way records are retrieved and manipulated.  They are generally created via the <tt>Database#from</tt> or <tt>Database#[]</tt> 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.  Datasets are always frozen, and they are safe to use by multiple threads concurrently.

=== Retrieving Records

You can retrieve all records by using the +all+ method:

  posts.all
  # 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 = posts.map([:name, :date])
  old_posts, recent_posts = posts.partition{|r| r[:date] < Date.today - 7}
  
You can also retrieve the first record in a dataset:

  posts.order(:id).first
  # SELECT * FROM posts ORDER BY id LIMIT 1
  
Note that you can get the first record in a dataset even if it isn't ordered:

  posts.first
  # SELECT * FROM posts LIMIT 1
  
If the dataset is ordered, you can also ask for the last record:

  posts.order(:stamp).last
  # SELECT * FROM posts ORDER BY stamp DESC LIMIT 1
  
You can also provide a filter when asking for a single record:

  posts.first(id: 1)
  # SELECT * FROM posts WHERE id = 1 LIMIT 1
  
Or retrieve a single value for a specific record:

  posts.where(id: 1).get(:name)
  # SELECT name FROM posts WHERE id = 1 LIMIT 1
  
=== Filtering Records

The most common 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: (Date.today - 14)..(Date.today - 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'))
  
By passing a block to where, you can use expressions (this is fairly "magical"):
  
  my_posts = posts.where{stamp > Date.today << 1}
  # WHERE (stamp > '2010-06-14')
  my_posts = posts.where{stamp =~ Date.today}
  # WHERE (stamp = '2010-07-14')
  
If you want to wrap the objects yourself, you can use expressions without the "magic":
  
  my_posts = posts.where(Sequel[:stamp] > Date.today << 1)
  # WHERE (stamp > '2010-06-14')
  my_posts = posts.where(Sequel[:stamp] =~ Date.today)
  # WHERE (stamp = '2010-07-14')
  
Some databases such as PostgreSQL and MySQL also support filtering via 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'))

But note that this does a full inversion of the filter:

  my_posts = posts.exclude(category: ['ruby', 'postgres', 'linux'], id: 1)
  # WHERE ((category NOT IN ('ruby', 'postgres', 'linux')) OR (id != 1))

If at any point you want to use a custom SQL fragment for part of a query,
you can do so via +Sequel.lit+:

  posts.where(Sequel.lit('stamp IS NOT NULL'))
  # WHERE (stamp IS NOT NULL)

You can safely interpolate parameters into the custom SQL fragment by
providing them as additional arguments:

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

Datasets can also be used as subqueries:

  DB[:items].where(Sequel[: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}[rdoc-ref:doc/dataset_filtering.rdoc] file for more details.

=== Security

Designing apps with security in mind is a best practice.
Please read the {Security Guide}[rdoc-ref:doc/security.rdoc] for details on security
issues that you should be aware of when using Sequel.

=== Summarizing Records

Counting records is easy using +count+:

  posts.where(Sequel.like(:category, '%ruby%')).count
  # SELECT COUNT(*) FROM posts WHERE (category LIKE '%ruby%' ESCAPE '\')

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+:

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

+order+ always overrides the existing order:

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

If you would like to add to the existing order, use +order_append+ or +order_prepend+:

  posts.order(:stamp).order_append(:name)
  # ORDER BY stamp, name
  posts.order(:stamp).order_prepend(:name)
  # ORDER BY name, stamp
  
You can also specify descending order:

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

=== Core Extensions

Note the use of <tt>Sequel.desc(:stamp)</tt> 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}[rdoc-ref:doc/core_extensions.rdoc] that integrates Sequel's DSL better into the Ruby language, allowing you to write:

  :stamp.desc

instead of:

  Sequel.desc(:stamp)

=== Selecting Columns

Selecting specific columns to be returned is also simple using +select+:

  posts.select(:stamp)
  # SELECT stamp FROM posts
  posts.select(:stamp, :name)
  # SELECT stamp, name FROM posts

Like +order+, +select+ overrides an existing selection:

  posts.select(:stamp).select(:name)
  # SELECT name FROM posts

As you might expect, there is an +order_append+ equivalent for +select+ called +select_append+:

  posts.select(:stamp).select_append(:name)
  # SELECT stamp, name FROM posts
  
=== Deleting Records

Deleting records from the table is done with +delete+:

  posts.where(Sequel[:stamp] < Date.today - 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:

  # DO THIS:
  posts.where(Sequel[:stamp] < Date.today - 7).delete
  # NOT THIS:
  posts.delete.where(Sequel[:stamp] < Date.today - 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(Sequel[:stamp] < Date.today - 7).update(state: 'archived')
  # UPDATE posts SET state = 'archived' WHERE (stamp < '2010-07-07')

You can provide arbitrary expressions when choosing what values to set:

  posts.where(Sequel[:stamp] < Date.today - 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:

  # DO THIS:
  posts.where(Sequel[:stamp] < Date.today - 7).update(state: 'archived')
  # NOT THIS:
  posts.update(state: 'archived').where(Sequel[:stamp] < Date.today - 7)

=== Merging records

Merging records using the SQL MERGE statement is done using <tt>merge*</tt> methods.
You use +merge_using+ to specify the merge source and join conditions.
You can use +merge_insert+, +merge_delete+, and/or +merge_update+ to set the
INSERT, DELETE, and UPDATE clauses for the merge. +merge_insert+ takes the same
arguments as +insert+, and +merge_update+ takes the same arguments as +update+.
+merge_insert+, +merge_delete+, and +merge_update+ can all be called with blocks,
to set the conditions for the related INSERT, DELETE, or UPDATE.

Finally, after calling all of the other <tt>merge_*</tt> methods, you call +merge+
to run the MERGE statement on the database.

  ds = DB[:m1]
    merge_using(:m2, i1: :i2).
    merge_insert(i1: :i2, a: Sequel[:b]+11).
    merge_delete{a > 30}.
    merge_update(i1: Sequel[:i1]+:i2+10, a: Sequel[:a]+:b+20)

  ds.merge
  # MERGE INTO m1 USING m2 ON (i1 = i2)
  # WHEN NOT MATCHED THEN INSERT (i1, a) VALUES (i2, (b + 11))
  # WHEN MATCHED AND (a > 30) THEN DELETE
  # WHEN MATCHED THEN UPDATE SET i1 = (i1 + i2 + 10), a = (a + b + 20)

=== Transactions

You can wrap a block of code in a database transaction using the <tt>Database#transaction</tt> method:

  DB.transaction do
    # BEGIN
    posts.insert(category: 'ruby', author: 'david')
    # INSERT
    posts.where(Sequel[:stamp] < Date.today - 7).update(state: 'archived')
    # UPDATE 
  end
  # COMMIT

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
<tt>Sequel::Rollback</tt> exception inside the block:

  DB.transaction do
    # BEGIN
    posts.insert(category: 'ruby', author: 'david')
    # INSERT
    if posts.where('stamp < ?', Date.today - 7).update(state: 'archived') == 0
    # UPDATE 
      raise Sequel::Rollback
    end
  end
  # 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 = items.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 = items.id)
  # WHERE (order_id = 1234)
  
Note that the default selection in Sequel is <tt>*</tt>, which includes all columns
in all joined tables.  Because Sequel returns results as a hash keyed by column name
symbols, if any tables have columns with the same name, this will clobber the columns
in the returned hash.  So when joining you are usually going to want to change the
selection using +select+, +select_all+, and/or +select_append+.

== 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.
The recommended way to qualify columns is to use <tt>Sequel[][]</tt> or +Sequel.qualify+

  Sequel[:table][:column]
  Sequel.qualify(:table, :column)
  # table.column

You can also qualify tables with schemas:

  Sequel[:schema][:table]
  # schema.table

or use multi-level qualification:

  Sequel[:schema][:table][:column]
  # schema.table.column

=== Expression aliases

You can alias identifiers using  <tt>Sequel[].as</tt> or +Sequel.as+:

  Sequel[:column].as(:alias)
  Sequel.as(:column, :alias)
  # column AS alias

You can use the <tt>Sequel.as</tt> method to alias arbitrary expressions, not just identifiers:

  Sequel.as(DB[:posts].select{max(id)}, :p)
  # (SELECT max(id) FROM posts) AS p

And most Sequel expression objects support an +as+ method for aliasing:

  (Sequel[:column] + 2).as(:c_plus_2)
  # (column + 2) AS c_plus_2

== 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 <tt>Sequel::Model</tt>:

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

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); end
  # or:
  class Post < Sequel::Model(DB[:my_posts]); end

If you pass a symbol to the <tt>Sequel::Model</tt> method, 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:

  class Post < Sequel::Model(DB[:my_posts].where(category: 'ruby')); end
  class Post < Sequel::Model(DB[:my_posts].select(:id, :name).order(:date)); end

=== Model instances

Model instances are identified by a primary key.  Sequel queries the database to determine the primary key for each model. The <tt>Model.[]</tt> 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:

  post.pk # => 123

If you want to override which column(s) to use as the primary key, you can use +set_primary_key+:

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

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

  Post.no_primary_key

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

  post = Post.first(title: 'hello world')
  post = Post.first{num_comments < 10}

The dataset for a model class returns rows of model instances instead of plain hashes:

  DB[:posts].first.class # => Hash
  Post.first.class # => Post

=== 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(Sequel.like(:title, /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:

  post.id # => 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 <tt>Model#[]</tt> to access the values:

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

You can also modify record values using attribute setters or the <tt>[]=</tt> 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:

  post.save

=== 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}[rdoc-ref:doc/mass_assignment.rdoc] 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 model instances can be created by calling <tt>Model.new</tt>, which returns a new model instance without updating the database:

  post = Post.new(title: 'hello world')

You can save the record to the database later by calling +save+ on the model instance:

  post.save

If you want to create a new record and save it to the database at the same time, you can use <tt>Model.create</tt>:

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

You can also supply a block to <tt>Model.new</tt> and <tt>Model.create</tt>:

  post = Post.new do |p|
    p.title = 'hello world'
  end

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

=== Hooks

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
      super
      author.increase_post_count
    end

    def after_destroy
      super
      author.decrease_post_count
    end
  end

Note the use of +super+ if you define your own hook methods.  Almost all <tt>Sequel::Model</tt> 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 database 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 <tt>delete</tt> and <tt>destroy</tt> 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 <tt>destroy</tt> is called, each record is deleted 
separately, but <tt>delete</tt> deletes all matching records with a single 
SQL query.

=== Associations

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
  end

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

  post = Post.create(name: 'hi!')
  post.author = Author.first(name: 'Sharon')
  post.author

+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!')
  post.comments

  comment = Comment.create(text: 'hi')
  post.add_comment(comment)
  post.remove_comment(comment)
  post.remove_all_comments

  tag = Tag.create(tag: 'interesting')
  post.add_tag(tag)
  post.remove_tag(tag)
  post.remove_all_tags
  
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
  post.comments_dataset.destroy

  # Return all tags related to this post with no subscribers, ordered by the tag's name
  post.tags_dataset.where(subscribers: 0).order(:name).all

=== Eager Loading

Associations can be eagerly loaded via +eager+ and the <tt>:eager</tt> 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]
  end

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

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

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

  # Eager loading via .eager
  Post.eager(:person).all

  # 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)
  person.posts
  
  # These are equivalent
  Post.eager(:person, :tags).all
  Post.eager(:person).eager(:tags).all
  
  # Cascading via .eager
  Tag.eager(posts: :replies).all
  
  # Will also grab all associated posts' tags (because of :eager)
  Reply.eager(person: :posts).all
  
  # 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 similar.  Note that using +eager_graph+ to eagerly load multiple <tt>*_to_many</tt> 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 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(Sequel.like(text, '%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(Sequel.like(text, '%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(Sequel.like(text, '%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 association:

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

This comes with variants for different join types:

  Post.association_left_join(:replies)
  # SELECT * FROM posts
  # LEFT JOIN replies ON (replies.post_id = posts.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 (author.id = posts.author_id)
  # INNER JOIN replies ON (replies.post_id = posts.id)
  # INNER JOIN people AS person ON (person.id = 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 with_few_comments
        where{num_comments < 30}
      end

      def clean_boring
        with_few_comments.delete
      end
    end
  end

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

  Post.where(category: 'ruby').clean_boring
  # DELETE FROM posts WHERE ((category = 'ruby') AND (num_comments < 30))

Inside +dataset_module+ blocks, there are numerous methods that support easy creation of dataset methods.
Most of these methods are named after the dataset methods themselves, such as +select+, +order+, and
+group+:

  class Post < Sequel::Model
    dataset_module do
      where(:with_few_comments, Sequel[:num_comments] < 30)
      select :with_title_and_date, :id, :title, :post_date
      order :by_post_date, :post_date
      limit :top10, 10
    end
  end

  Post.with_few_comments.with_title_and_date.by_post_date.top10
  # SELECT id, title, post_date
  # FROM posts
  # ORDER BY post_date
  # LIMIT 10

One advantage of using these methods inside dataset_module blocks, instead of
defining methods manually, is that the created methods will generally cache
the resulting values and result in better performance.

=== 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 an 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 by default:

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

== Testing Sequel

Please see the {testing guide}[rdoc-ref:doc/testing.rdoc] for recommendations on testing
applications that use Sequel, as well as the how to run the tests for Sequel itself.

== Sequel Release Policy

New major versions of Sequel do not have a defined release policy, but historically have
occurred once every few years.

New minor versions of Sequel are released around once a month near the start of the month.

New tiny versions of Sequel are only released to address security issues or regressions
in the most current release.

== Ruby Support Policy

Sequel fully supports the currently supported versions of Ruby (MRI) and JRuby.  It may
support unsupported versions of Ruby or JRuby, but such support may be dropped in any
minor version if keeping it becomes a support issue.  The minimum Ruby version
required to run the current version of Sequel is 1.9.2, and the minimum JRuby version is
9.2.0.0 (due to the bigdecimal dependency).

== Maintainer

Jeremy Evans <code@jeremyevans.net>
