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<option value="3_2_release_notes.html">Version 3.2 - January 2012</option> <option value="3_1_release_notes.html">Version 3.1 - August 2011</option> <option value="3_0_release_notes.html">Version 3.0 - August 2010</option> <option value="2_3_release_notes.html">Version 2.3 - March 2009</option> <option value="2_2_release_notes.html">Version 2.2 - November 2008</option> </optgroup> </select> </li> </ul> </nav> </div> </header> <hr class="hide" /> <main id="main"> <article> <header id="feature"> <div class="wrapper"> <h1>Active Record Basics</h1>This guide is an introduction to Active Record.After reading this guide, you will know: <ul> <li>How Active Record fits into the Model-View-Controller (MVC) paradigm.</li> <li>What Object Relational Mapping and Active Record patterns are and how they are used in Rails.</li> <li>How to use Active Record models to manipulate data stored in a relational database.</li> <li>Active Record schema naming conventions.</li> <li>The concepts of database migrations, validations, callbacks, and associations.</li> </ul> <nav id="column-side" aria-label="Chapter" class="guide-index" data-turbo="false"> <a id="chapter-nav-skip-link" href="#article-body" class="skip-link"> Skip to article body </a> <h2 class="chapter"> <picture aria-hidden="true">  <source srcset="images/icon_book-close-bookmark-1-wht.svg" media="(prefers-color-scheme: dark)" /> <img src="images/icon_book-close-bookmark-1.svg" alt="Chapter Icon" /> </picture> Chapters </h2> <ol class="chapters"> <li><a href="#what-is-active-record-questionmark">What is Active Record?</a> <ul> <li><a href="#the-active-record-pattern">The Active Record Pattern</a></li> <li><a href="#object-relational-mapping">Object Relational Mapping</a></li> <li><a href="#active-record-as-an-orm-framework">Active Record as an ORM Framework</a></li> </ul></li> <li><a href="#convention-over-configuration-in-active-record">Convention over Configuration in Active Record</a> <ul> <li><a href="#naming-conventions">Naming Conventions</a></li> <li><a href="#schema-conventions">Schema Conventions</a></li> </ul></li> <li><a href="#creating-active-record-models">Creating Active Record Models</a> <ul> <li><a href="#creating-namespaced-models">Creating Namespaced Models</a></li> </ul></li> <li><a href="#overriding-the-naming-conventions">Overriding the Naming Conventions</a></li> <li><a href="#crud-reading-and-writing-data">CRUD: Reading and Writing Data</a> <ul> <li><a href="#create">Create</a></li> <li><a href="#read">Read</a></li> <li><a href="#update">Update</a></li> <li><a href="#delete">Delete</a></li> </ul></li> <li><a href="#validations">Validations</a></li> <li><a href="#callbacks">Callbacks</a></li> <li><a href="#migrations">Migrations</a></li> <li><a href="#associations">Associations</a></li> </ol> </nav> </div> </header> <div class="wrapper"> <div id="column-main"> <section id="article-body"> <h2 id="what-is-active-record-questionmark"><a class="anchorlink" href="#what-is-active-record-questionmark" data-turbo="false">1 What is Active Record?</a></h2>Active Record is part of the M in <a href="https://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller">MVC</a> - the model - which is the layer of the system responsible for representing data and business logic. Active Record helps you create and use Ruby objects whose attributes require persistent storage to a database.<div class="interstitial note">What is the difference between Active Record and Active Model? It's possible to model data with Ruby objects that do not need to be backed by a database. <a href="active_model_basics.html">Active Model</a> is commonly used for that in Rails, making Active Record and Active Model both part of the M in MVC, as well as your own plain Ruby objects.</div>The term "Active Record" also refers to a software architecture pattern. Active Record in Rails is an implementation of that pattern. It's also a description of something called an <a href="https://en.wikipedia.org/wiki/Object-relational_mapping">Object Relational Mapping</a> system. The below sections explain these terms.<h3 id="the-active-record-pattern"><a class="anchorlink" href="#the-active-record-pattern" data-turbo="false">1.1 The Active Record Pattern</a></h3>The <a href="https://www.martinfowler.com/eaaCatalog/activeRecord.html">Active Record pattern is described by Martin Fowler</a> in the book Patterns of Enterprise Application Architecture as "an object that wraps a row in a database table, encapsulates the database access, and adds domain logic to that data." Active Record objects carry both data and behavior. Active Record classes match very closely to the record structure of the underlying database. This way users can easily read from and write to the database, as you will see in the examples below.<h3 id="object-relational-mapping"><a class="anchorlink" href="#object-relational-mapping" data-turbo="false">1.2 Object Relational Mapping</a></h3>Object Relational Mapping, commonly referred to as ORM, is a technique that connects the rich objects of a programming language to tables in a relational database management system (RDBMS). In the case of a Rails application, these are Ruby objects. Using an ORM, the attributes of Ruby objects, as well as the relationship between objects, can be easily stored and retrieved from a database without writing SQL statements directly. Overall, ORMs minimize the amount of database access code you have to write.<div class="interstitial note">Basic knowledge of relational database management systems (RDBMS) and structured query language (SQL) is helpful in order to fully understand Active Record. Please refer to <a href="https://www.khanacademy.org/computing/computer-programming/sql">this SQL tutorial</a> (or <a href="https://www.devart.com/what-is-rdbms/">this RDBMS tutorial</a>) or study them by other means if you would like to learn more.</div><h3 id="active-record-as-an-orm-framework"><a class="anchorlink" href="#active-record-as-an-orm-framework" data-turbo="false">1.3 Active Record as an ORM Framework</a></h3>Active Record gives us the ability to do the following using Ruby objects: <ul> <li>Represent models and their data.</li> <li>Represent associations between models.</li> <li>Represent inheritance hierarchies through related models.</li> <li>Validate models before they get persisted to the database.</li> <li>Perform database operations in an object-oriented fashion.</li> </ul> <h2 id="convention-over-configuration-in-active-record"><a class="anchorlink" href="#convention-over-configuration-in-active-record" data-turbo="false">2 Convention over Configuration in Active Record</a></h2>When writing applications using other programming languages or frameworks, it may be necessary to write a lot of configuration code. This is particularly true for ORM frameworks in general. However, if you follow the conventions adopted by Rails, you'll write very little to no configuration code when creating Active Record models.Rails adopts the idea that if you configure your applications in the same way most of the time, then that way should be the default. Explicit configuration should be needed only in those cases where you can't follow the convention.To take advantage of convention over configuration in Active Record, there are some naming and schema conventions to follow. And in case you need to, it is possible to <a href="#overriding-the-naming-conventions">override naming conventions</a>.<h3 id="naming-conventions"><a class="anchorlink" href="#naming-conventions" data-turbo="false">2.1 Naming Conventions</a></h3>Active Record uses this naming convention to map between models (represented by Ruby objects) and database tables:Rails will pluralize your model's class names to find the respective database table. For example, a class named <code>Book</code> maps to a database table named <code>books</code>. The Rails pluralization mechanisms are very powerful and capable of pluralizing (and singularizing) both regular and irregular words in the English language. This uses the <a href="active_support_core_extensions.html#pluralize">Active Support</a> <a href="https://api.rubyonrails.org/v8.0.0/classes/ActiveSupport/Inflector.html#method-i-pluralize">pluralize</a> method.For class names composed of two or more words, the model class name will follow the Ruby conventions of using an UpperCamelCase name. The database table name, in that case, will be a snake_case name. For example: <ul> <li><code>BookClub</code> is the model class, singular with the first letter of each word capitalized.</li> <li><code>book_clubs</code> is the matching database table, plural with underscores separating words.</li> </ul> Here are some more examples of model class names and corresponding table names: <div class="table-wrapper"><table><thead> <tr> <th>Model / Class</th> <th>Table / Schema</th> </tr> </thead><tbody> <tr> <td><code>Article</code></td> <td><code>articles</code></td> </tr> <tr> <td><code>LineItem</code></td> <td><code>line_items</code></td> </tr> <tr> <td><code>Product</code></td> <td><code>products</code></td> </tr> <tr> <td><code>Person</code></td> <td><code>people</code></td> </tr> </tbody></table></div> <h3 id="schema-conventions"><a class="anchorlink" href="#schema-conventions" data-turbo="false">2.2 Schema Conventions</a></h3>Active Record uses conventions for column names in the database tables as well, depending on the purpose of these columns. <ul> <li>Primary keys - By default, Active Record will use an integer column named <code>id</code> as the table's primary key (<code>bigint</code> for PostgreSQL, MySQL, and MariaDB, <code>integer</code> for SQLite). When using <a href="#migrations">Active Record Migrations</a> to create your tables, this column will be automatically created.</li> <li>Foreign keys - These fields should be named following the pattern <code>singularized_table_name_id</code> (e.g., <code>order_id</code>, <code>line_item_id</code>). These are the fields that Active Record will look for when you create associations between your models.</li> </ul> There are also some optional column names that will add additional features to Active Record instances: <ul> <li><code>created_at</code> - Automatically gets set to the current date and time when the record is first created.</li> <li><code>updated_at</code> - Automatically gets set to the current date and time whenever the record is created or updated.</li> <li><code>lock_version</code> - Adds <a href="https://api.rubyonrails.org/v8.0.0/classes/ActiveRecord/Locking.html">optimistic locking</a> to a model.</li> <li><code>type</code> - Specifies that the model uses <a href="https://api.rubyonrails.org/v8.0.0/classes/ActiveRecord/Base.html#class-ActiveRecord::Base-label-Single+table+inheritance">Single Table Inheritance</a>.</li> <li><code>(association_name)_type</code> - Stores the type for <a href="association_basics.html#polymorphic-associations">polymorphic associations</a>.</li> <li><code>(table_name)_count</code> - Used to cache the number of belonging objects on associations. For example, if <code>Article</code>s have many <code>Comment</code>s, a <code>comments_count</code> column in the <code>articles</code> table will cache the number of existing comments for each article.</li> </ul> <div class="interstitial note">While these column names are optional, they are reserved by Active Record. Steer clear of reserved keywords when naming your table's columns. For example, <code>type</code> is a reserved keyword used to designate a table using Single Table Inheritance (STI). If you are not using STI, use a different word to accurately describe the data you are modeling.</div><h2 id="creating-active-record-models"><a class="anchorlink" href="#creating-active-record-models" data-turbo="false">3 Creating Active Record Models</a></h2>When generating a Rails application, an abstract <code>ApplicationRecord</code> class will be created in <code>app/models/application_record.rb</code>. The <code>ApplicationRecord</code> class inherits from <a href="https://api.rubyonrails.org/v8.0.0/classes/ActiveRecord/Base.html"><code>ActiveRecord::Base</code></a> and it's what turns a regular Ruby class into an Active Record model.<code>ApplicationRecord</code> is the base class for all Active Record models in your app. To create a new model, subclass the <code>ApplicationRecord</code> class and you're good to go:<div class="interstitial code"> <pre><code class="highlight ruby">class Book < ApplicationRecord end </code></pre> <button class="clipboard-button" data-clipboard-text="class Book < ApplicationRecord end ">Copy</button> </div> This will create a <code>Book</code> model, mapped to a <code>books</code> table in the database, where each column in the table is mapped to attributes of the <code>Book</code> class. An instance of <code>Book</code> can represent a row in the <code>books</code> table. The <code>books</code> table with columns <code>id</code>, <code>title</code>, and <code>author</code>, can be created using an SQL statement like this:<div class="interstitial code"> <pre><code class="highlight sql">CREATE TABLE books ( id int(11) NOT NULL auto_increment, title varchar(255), author varchar(255), PRIMARY KEY (id) ); </code></pre> <button class="clipboard-button" data-clipboard-text="CREATE TABLE books ( id int(11) NOT NULL auto_increment, title varchar(255), author varchar(255), PRIMARY KEY (id) ); ">Copy</button> </div> However, that is not how you do it normally in Rails. Database tables in Rails are typically created using <a href="#migrations">Active Record Migrations</a> and not raw SQL. A migration for the <code>books</code> table above can be generated like this:<div class="interstitial code"> <pre><code class="highlight console">$ bin/rails generate migration CreateBooks title:string author:string </code></pre> <button class="clipboard-button" data-clipboard-text="bin/rails generate migration CreateBooks title:string author:string ">Copy</button> </div> and results in this:<div class="interstitial code"> <pre><code class="highlight ruby"># Note: # The `id` column, as the primary key, is automatically created by convention. # Columns `created_at` and `updated_at` are added by `t.timestamps`. # db/migrate/20240220143807_create_books.rb class CreateBooks < ActiveRecord::Migration[8.0] def change create_table :books do |t| t.string :title t.string :author t.timestamps end end end </code></pre> <button class="clipboard-button" data-clipboard-text="# Note: # The `id` column, as the primary key, is automatically created by convention. # Columns `created_at` and `updated_at` are added by `t.timestamps`. class CreateBooks < ActiveRecord::Migration[8.0] def change create_table :books do |t| t.string :title t.string :author t.timestamps end end end ">Copy</button> </div> That migration creates columns <code>id</code>, <code>title</code>, <code>author</code>, <code>created_at</code> and <code>updated_at</code>. Each row of this table can be represented by an instance of the <code>Book</code> class with the same attributes: <code>id</code>, <code>title</code>, <code>author</code>, <code>created_at</code>, and <code>updated_at</code>. You can access a book's attributes like this:<div class="interstitial code"> <pre><code class="highlight irb">irb> book = Book.new => #<Book:0x00007fbdf5e9a038 id: nil, title: nil, author: nil, created_at: nil, updated_at: nil> irb> book.title = "The Hobbit" => "The Hobbit" irb> book.title => "The Hobbit" </code></pre> <button class="clipboard-button" data-clipboard-text="book = Book.new book.title = "The Hobbit" book.title ">Copy</button> </div> <div class="interstitial note">You can generate the Active Record model class as well as a matching migration with the command <code>bin/rails generate model Book title:string author:string</code>. This creates the files <code>app/models/book.rb</code>, <code>db/migrate/20240220143807_create_books.rb</code>, and a couple others for testing purposes.</div><h3 id="creating-namespaced-models"><a class="anchorlink" href="#creating-namespaced-models" data-turbo="false">3.1 Creating Namespaced Models</a></h3>Active Record models are placed under the <code>app/models</code> directory by default. But you may want to organize your models by placing similar models under their own folder and namespace. For example, <code>order.rb</code> and <code>review.rb</code> under <code>app/models/book</code> with <code>Book::Order</code> and <code>Book::Review</code> class names, respectively. You can create namespaced models with Active Record.In the case where the <code>Book</code> module does not already exist, the <code>generate</code> command will create everything like this:<div class="interstitial code"> <pre><code class="highlight console">$ bin/rails generate model Book::Order invoke active_record create db/migrate/20240306194227_create_book_orders.rb create app/models/book/order.rb create app/models/book.rb invoke test_unit create test/models/book/order_test.rb create test/fixtures/book/orders.yml </code></pre> <button class="clipboard-button" data-clipboard-text="bin/rails generate model Book::Order ">Copy</button> </div> If the <code>Book</code> module already exists, you will be asked to resolve the conflict:<div class="interstitial code"> <pre><code class="highlight console">$ bin/rails generate model Book::Order invoke active_record create db/migrate/20240305140356_create_book_orders.rb create app/models/book/order.rb conflict app/models/book.rb Overwrite /Users/bhumi/Code/rails_guides/app/models/book.rb? (enter "h" for help) [Ynaqdhm] </code></pre> <button class="clipboard-button" data-clipboard-text="bin/rails generate model Book::Order ">Copy</button> </div> Once the namespaced model generation is successful, the <code>Book</code> and <code>Order</code> classes look like this:<div class="interstitial code"> <pre><code class="highlight ruby"># app/models/book.rb module Book def self.table_name_prefix "book_" end end # app/models/book/order.rb class Book::Order < ApplicationRecord end </code></pre> <button class="clipboard-button" data-clipboard-text="module Book def self.table_name_prefix "book_" end end class Book::Order < ApplicationRecord end ">Copy</button> </div> Setting the <a href="https://api.rubyonrails.org/v8.0.0/classes/ActiveRecord/ModelSchema.html#method-c-table_name_prefix-3D">table_name_prefix</a> in <code>Book</code> will allow <code>Order</code> model's database table to be named <code>book_orders</code>, instead of plain <code>orders</code>.The other possibility is that you already have a <code>Book</code> model that you want to keep in <code>app/models</code>. In that case, you can choose <code>n</code> to not overwrite <code>book.rb</code> during the <code>generate</code> command.This will still allow for a namespaced table name for <code>Book::Order</code> class, without needing the <code>table_name_prefix</code>:<div class="interstitial code"> <pre><code class="highlight ruby"># app/models/book.rb class Book < ApplicationRecord # existing code end Book::Order.table_name # => "book_orders" </code></pre> <button class="clipboard-button" data-clipboard-text="class Book < ApplicationRecord # existing code end Book::Order.table_name # => "book_orders" ">Copy</button> </div> <h2 id="overriding-the-naming-conventions"><a class="anchorlink" href="#overriding-the-naming-conventions" data-turbo="false">4 Overriding the Naming Conventions</a></h2>What if you need to follow a different naming convention or need to use your Rails application with a legacy database? No problem, you can easily override the default conventions.Since <code>ApplicationRecord</code> inherits from <code>ActiveRecord::Base</code>, your application's models will have a number of helpful methods available to them. For example, you can use the <code>ActiveRecord::Base.table_name=</code> method to customize the table name that should be used:<div class="interstitial code"> <pre><code class="highlight ruby">class Book < ApplicationRecord self.table_name = "my_books" end </code></pre> <button class="clipboard-button" data-clipboard-text="class Book < ApplicationRecord self.table_name = "my_books" end ">Copy</button> </div> If you do so, you will have to manually define the class name that is hosting <a href="testing.html#the-low-down-on-fixtures">the fixtures</a> (<code>my_books.yml</code>) using the <code>set_fixture_class</code> method in your test definition:<div class="interstitial code"> <pre><code class="highlight ruby"># test/models/book_test.rb class BookTest < ActiveSupport::TestCase set_fixture_class my_books: Book fixtures :my_books # ... end </code></pre> <button class="clipboard-button" data-clipboard-text="class BookTest < ActiveSupport::TestCase set_fixture_class my_books: Book fixtures :my_books # ... end ">Copy</button> </div> It's also possible to override the column that should be used as the table's primary key using the <code>ActiveRecord::Base.primary_key=</code> method:<div class="interstitial code"> <pre><code class="highlight ruby">class Book < ApplicationRecord self.primary_key = "book_id" end </code></pre> <button class="clipboard-button" data-clipboard-text="class Book < ApplicationRecord self.primary_key = "book_id" end ">Copy</button> </div> <div class="interstitial note">Active Record does not recommend using non-primary key columns named <code>id</code>. Using a column named <code>id</code> which is not a single-column primary key complicates the access to the column value. The application will have to use the <a href="https://api.rubyonrails.org/v8.0.0/classes/ActiveRecord/ModelSchema.html#method-i-id_value"><code>id_value</code></a> alias attribute to access the value of the non-PK <code>id</code> column.</div><div class="interstitial note">If you try to create a column named <code>id</code> which is not the primary key, Rails will throw an error during migrations such as: <code>you can't redefine the primary key column 'id' on 'my_books'.</code> <code>To define a custom primary key, pass { id: false } to create_table.</code></div><h2 id="crud-reading-and-writing-data"><a class="anchorlink" href="#crud-reading-and-writing-data" data-turbo="false">5 CRUD: Reading and Writing Data</a></h2>CRUD is an acronym for the four verbs we use to operate on data: Create, Read, Update, and Delete. Active Record automatically creates methods to allow you to read and manipulate data stored in your application's database tables.Active Record makes it seamless to perform CRUD operations by using these high-level methods that abstract away database access details. Note that all of these convenient methods result in SQL statement(s) that are executed against the underlying database.The examples below show a few of the CRUD methods as well as the resulting SQL statements.<h3 id="create"><a class="anchorlink" href="#create" data-turbo="false">5.1 Create</a></h3>Active Record objects can be created from a hash, a block, or have their attributes manually set after creation. The <code>new</code> method will return a new, non-persisted object, while <code>create</code> will save the object to the database and return it.For example, given a <code>Book</code> model with attributes of <code>title</code> and <code>author</code>, the <code>create</code> method call will create an object and save a new record to the database:<div class="interstitial code"> <pre><code class="highlight ruby">book = Book.create(title: "The Lord of the Rings", author: "J.R.R. Tolkien") # Note that the `id` is assigned as this record is committed to the database. book.inspect # => "#<Book id: 106, title: \"The Lord of the Rings\", author: \"J.R.R. Tolkien\", created_at: \"2024-03-04 19:15:58.033967000 +0000\", updated_at: \"2024-03-04 19:15:58.033967000 +0000\">" </code></pre> <button class="clipboard-button" data-clipboard-text="book = Book.create(title: "The Lord of the Rings", author: "J.R.R. Tolkien") # Note that the `id` is assigned as this record is committed to the database. book.inspect # => "#<Book id: 106, title: \"The Lord of the Rings\", author: \"J.R.R. Tolkien\", created_at: \"2024-03-04 19:15:58.033967000 +0000\", updated_at: \"2024-03-04 19:15:58.033967000 +0000\">" ">Copy</button> </div> While the <code>new</code> method will instantiate an object without saving it to the database:<div class="interstitial code"> <pre><code class="highlight ruby">book = Book.new book.title = "The Hobbit" book.author = "J.R.R. Tolkien" # Note that the `id` is not set for this object. book.inspect # => "#<Book id: nil, title: \"The Hobbit\", author: \"J.R.R. Tolkien\", created_at: nil, updated_at: nil>" # The above `book` is not yet saved to the database. book.save book.id # => 107 # Now the `book` record is committed to the database and has an `id`. </code></pre> <button class="clipboard-button" data-clipboard-text="book = Book.new book.title = "The Hobbit" book.author = "J.R.R. Tolkien" # Note that the `id` is not set for this object. book.inspect # => "#<Book id: nil, title: \"The Hobbit\", author: \"J.R.R. Tolkien\", created_at: nil, updated_at: nil>" # The above `book` is not yet saved to the database. book.save book.id # => 107 # Now the `book` record is committed to the database and has an `id`. ">Copy</button> </div> Finally, if a block is provided, both <code>create</code> and <code>new</code> will yield the new object to that block for initialization, while only <code>create</code> will persist the resulting object to the database:<div class="interstitial code"> <pre><code class="highlight ruby">book = Book.new do |b| b.title = "Metaprogramming Ruby 2" b.author = "Paolo Perrotta" end book.save </code></pre> <button class="clipboard-button" data-clipboard-text="book = Book.new do |b| b.title = "Metaprogramming Ruby 2" b.author = "Paolo Perrotta" end book.save ">Copy</button> </div> The resulting SQL statement from both <code>book.save</code> and <code>Book.create</code> look something like this:<div class="interstitial code"> <pre><code class="highlight sql">/* Note that `created_at` and `updated_at` are automatically set. */ INSERT INTO "books" ("title", "author", "created_at", "updated_at") VALUES (?, ?, ?, ?) RETURNING "id" [["title", "Metaprogramming Ruby 2"], ["author", "Paolo Perrotta"], ["created_at", "2024-02-22 20:01:18.469952"], ["updated_at", "2024-02-22 20:01:18.469952"]] </code></pre> <button class="clipboard-button" data-clipboard-text="/* Note that `created_at` and `updated_at` are automatically set. */ INSERT INTO "books" ("title", "author", "created_at", "updated_at") VALUES (?, ?, ?, ?) RETURNING "id" [["title", "Metaprogramming Ruby 2"], ["author", "Paolo Perrotta"], ["created_at", "2024-02-22 20:01:18.469952"], ["updated_at", "2024-02-22 20:01:18.469952"]] ">Copy</button> </div> <h3 id="read"><a class="anchorlink" href="#read" data-turbo="false">5.2 Read</a></h3>Active Record provides a rich API for accessing data within a database. You can query a single record or multiple records, filter them by any attribute, order them, group them, select specific fields, and do anything you can do with SQL.<div class="interstitial code"> <pre><code class="highlight ruby"># Return a collection with all books. books = Book.all # Return a single book. first_book = Book.first last_book = Book.last book = Book.take </code></pre> <button class="clipboard-button" data-clipboard-text="# Return a collection with all books. books = Book.all # Return a single book. first_book = Book.first last_book = Book.last book = Book.take ">Copy</button> </div> The above results in the following SQL:<div class="interstitial code"> <pre><code class="highlight sql">-- Book.all SELECT "books".* FROM "books" -- Book.first SELECT "books".* FROM "books" ORDER BY "books"."id" ASC LIMIT ? [["LIMIT", 1]] -- Book.last SELECT "books".* FROM "books" ORDER BY "books"."id" DESC LIMIT ? [["LIMIT", 1]] -- Book.take SELECT "books".* FROM "books" LIMIT ? [["LIMIT", 1]] </code></pre> <button class="clipboard-button" data-clipboard-text="-- Book.all SELECT "books".* FROM "books" -- Book.first SELECT "books".* FROM "books" ORDER BY "books"."id" ASC LIMIT ? [["LIMIT", 1]] -- Book.last SELECT "books".* FROM "books" ORDER BY "books"."id" DESC LIMIT ? [["LIMIT", 1]] -- Book.take SELECT "books".* FROM "books" LIMIT ? [["LIMIT", 1]] ">Copy</button> </div> We can also find specific books with <code>find_by</code> and <code>where</code>. While <code>find_by</code> returns a single record, <code>where</code> returns a list of records:<div class="interstitial code"> <pre><code class="highlight ruby"># Returns the first book with a given title or `nil` if no book is found. book = Book.find_by(title: "Metaprogramming Ruby 2") # Alternative to Book.find_by(id: 42). Will throw an exception if no matching book is found. book = Book.find(42) </code></pre> <button class="clipboard-button" data-clipboard-text="# Returns the first book with a given title or `nil` if no book is found. book = Book.find_by(title: "Metaprogramming Ruby 2") # Alternative to Book.find_by(id: 42). Will throw an exception if no matching book is found. book = Book.find(42) ">Copy</button> </div> The above resulting in this SQL:<div class="interstitial code"> <pre><code class="highlight sql">-- Book.find_by(title: "Metaprogramming Ruby 2") SELECT "books".* FROM "books" WHERE "books"."title" = ? LIMIT ? [["title", "Metaprogramming Ruby 2"], ["LIMIT", 1]] -- Book.find(42) SELECT "books".* FROM "books" WHERE "books"."id" = ? LIMIT ? [["id", 42], ["LIMIT", 1]] </code></pre> <button class="clipboard-button" data-clipboard-text="-- Book.find_by(title: "Metaprogramming Ruby 2") SELECT "books".* FROM "books" WHERE "books"."title" = ? LIMIT ? [["title", "Metaprogramming Ruby 2"], ["LIMIT", 1]] -- Book.find(42) SELECT "books".* FROM "books" WHERE "books"."id" = ? LIMIT ? [["id", 42], ["LIMIT", 1]] ">Copy</button> </div> <div class="interstitial code"> <pre><code class="highlight ruby"># Find all books by a given author, sort by created_at in reverse chronological order. Book.where(author: "Douglas Adams").order(created_at: :desc) </code></pre> <button class="clipboard-button" data-clipboard-text="# Find all books by a given author, sort by created_at in reverse chronological order. Book.where(author: "Douglas Adams").order(created_at: :desc) ">Copy</button> </div> resulting in this SQL:<div class="interstitial code"> <pre><code class="highlight sql">SELECT "books".* FROM "books" WHERE "books"."author" = ? ORDER BY "books"."created_at" DESC [["author", "Douglas Adams"]] </code></pre> <button class="clipboard-button" data-clipboard-text="SELECT "books".* FROM "books" WHERE "books"."author" = ? ORDER BY "books"."created_at" DESC [["author", "Douglas Adams"]] ">Copy</button> </div> There are many more Active Record methods to read and query records. You can learn more about them in the <a href="active_record_querying.html">Active Record Query</a> guide.<h3 id="update"><a class="anchorlink" href="#update" data-turbo="false">5.3 Update</a></h3>Once an Active Record object has been retrieved, its attributes can be modified and it can be saved to the database.<div class="interstitial code"> <pre><code class="highlight ruby">book = Book.find_by(title: "The Lord of the Rings") book.title = "The Lord of the Rings: The Fellowship of the Ring" book.save </code></pre> <button class="clipboard-button" data-clipboard-text="book = Book.find_by(title: "The Lord of the Rings") book.title = "The Lord of the Rings: The Fellowship of the Ring" book.save ">Copy</button> </div> A shorthand for this is to use a hash mapping attribute names to the desired value, like so:<div class="interstitial code"> <pre><code class="highlight ruby">book = Book.find_by(title: "The Lord of the Rings") book.update(title: "The Lord of the Rings: The Fellowship of the Ring") </code></pre> <button class="clipboard-button" data-clipboard-text="book = Book.find_by(title: "The Lord of the Rings") book.update(title: "The Lord of the Rings: The Fellowship of the Ring") ">Copy</button> </div> the <code>update</code> results in the following SQL:<div class="interstitial code"> <pre><code class="highlight sql">/* Note that `updated_at` is automatically set. */ UPDATE "books" SET "title" = ?, "updated_at" = ? WHERE "books"."id" = ? [["title", "The Lord of the Rings: The Fellowship of the Ring"], ["updated_at", "2024-02-22 20:51:13.487064"], ["id", 104]] </code></pre> <button class="clipboard-button" data-clipboard-text="/* Note that `updated_at` is automatically set. */ UPDATE "books" SET "title" = ?, "updated_at" = ? WHERE "books"."id" = ? [["title", "The Lord of the Rings: The Fellowship of the Ring"], ["updated_at", "2024-02-22 20:51:13.487064"], ["id", 104]] ">Copy</button> </div> This is useful when updating several attributes at once. Similar to <code>create</code>, using <code>update</code> will commit the updated records to the database.If you'd like to update several records in bulk without callbacks or validations, you can update the database directly using <code>update_all</code>:<div class="interstitial code"> <pre><code class="highlight ruby">Book.update_all(status: "already own") </code></pre> <button class="clipboard-button" data-clipboard-text="Book.update_all(status: "already own") ">Copy</button> </div> <h3 id="delete"><a class="anchorlink" href="#delete" data-turbo="false">5.4 Delete</a></h3>Likewise, once retrieved, an Active Record object can be destroyed, which removes it from the database.<div class="interstitial code"> <pre><code class="highlight ruby">book = Book.find_by(title: "The Lord of the Rings") book.destroy </code></pre> <button class="clipboard-button" data-clipboard-text="book = Book.find_by(title: "The Lord of the Rings") book.destroy ">Copy</button> </div> The <code>destroy</code> results in this SQL:<div class="interstitial code"> <pre><code class="highlight sql">DELETE FROM "books" WHERE "books"."id" = ? [["id", 104]] </code></pre> <button class="clipboard-button" data-clipboard-text="DELETE FROM "books" WHERE "books"."id" = ? [["id", 104]] ">Copy</button> </div> If you'd like to delete several records in bulk, you may use <code>destroy_by</code> or <code>destroy_all</code> method:<div class="interstitial code"> <pre><code class="highlight ruby"># Find and delete all books by Douglas Adams. Book.destroy_by(author: "Douglas Adams") # Delete all books. Book.destroy_all </code></pre> <button class="clipboard-button" data-clipboard-text="# Find and delete all books by Douglas Adams. Book.destroy_by(author: "Douglas Adams") # Delete all books. Book.destroy_all ">Copy</button> </div> <h2 id="validations"><a class="anchorlink" href="#validations" data-turbo="false">6 Validations</a></h2>Active Record allows you to validate the state of a model before it gets written into the database. There are several methods that allow for different types of validations. For example, validate that an attribute value is not empty, is unique, is not already in the database, follows a specific format, and many more.Methods like <code>save</code>, <code>create</code> and <code>update</code> validate a model before persisting it to the database. If the model is invalid, no database operations are performed. In this case the <code>save</code> and <code>update</code> methods return <code>false</code>. The <code>create</code> method still returns the object, which can be checked for errors. All of these methods have a bang counterpart (that is, <code>save!</code>, <code>create!</code> and <code>update!</code>), which are stricter in that they raise an <code>ActiveRecord::RecordInvalid</code> exception when validation fails. A quick example to illustrate:<div class="interstitial code"> <pre><code class="highlight ruby">class User < ApplicationRecord validates :name, presence: true end </code></pre> <button class="clipboard-button" data-clipboard-text="class User < ApplicationRecord validates :name, presence: true end ">Copy</button> </div> <div class="interstitial code"> <pre><code class="highlight irb">irb> user = User.new irb> user.save => false irb> user.save! ActiveRecord::RecordInvalid: Validation failed: Name can't be blank </code></pre> <button class="clipboard-button" data-clipboard-text="user = User.new user.save user.save! ">Copy</button> </div> The <code>create</code> method always returns the model, regardless of its validity. You can then inspect this model for any errors.<div class="interstitial code"> <pre><code class="highlight irb">irb> user = User.create => #<User:0x000000013e8b5008 id: nil, name: nil> irb> user.errors.full_messages => ["Name can't be blank"] </code></pre> <button class="clipboard-button" data-clipboard-text="user = User.create user.errors.full_messages ">Copy</button> </div> You can learn more about validations in the <a href="active_record_validations.html">Active Record Validations guide</a>.<h2 id="callbacks"><a class="anchorlink" href="#callbacks" data-turbo="false">7 Callbacks</a></h2>Active Record callbacks allow you to attach code to certain events in the lifecycle of your models. This enables you to add behavior to your models by executing code when those events occur, like when you create a new record, update it, destroy it, and so on.<div class="interstitial code"> <pre><code class="highlight ruby">class User < ApplicationRecord after_create :log_new_user private def log_new_user puts "A new user was registered" end end </code></pre> <button class="clipboard-button" data-clipboard-text="class User < ApplicationRecord after_create :log_new_user private def log_new_user puts "A new user was registered" end end ">Copy</button> </div> <div class="interstitial code"> <pre><code class="highlight irb">irb> @user = User.create A new user was registered </code></pre> <button class="clipboard-button" data-clipboard-text="@user = User.create ">Copy</button> </div> You can learn more about callbacks in the <a href="active_record_callbacks.html">Active Record Callbacks guide</a>.<h2 id="migrations"><a class="anchorlink" href="#migrations" data-turbo="false">8 Migrations</a></h2>Rails provides a convenient way to manage changes to a database schema via migrations. Migrations are written in a domain-specific language and stored in files which are executed against any database that Active Record supports.Here's a migration that creates a new table called <code>publications</code>:<div class="interstitial code"> <pre><code class="highlight ruby">class CreatePublications < ActiveRecord::Migration[8.0] def change create_table :publications do |t| t.string :title t.text :description t.references :publication_type t.references :publisher, polymorphic: true t.boolean :single_issue t.timestamps end end end </code></pre> <button class="clipboard-button" data-clipboard-text="class CreatePublications < ActiveRecord::Migration[8.0] def change create_table :publications do |t| t.string :title t.text :description t.references :publication_type t.references :publisher, polymorphic: true t.boolean :single_issue t.timestamps end end end ">Copy</button> </div> Note that the above code is database-agnostic: it will run in MySQL, MariaDB, PostgreSQL, SQLite, and others.Rails keeps track of which migrations have been committed to the database and stores them in a neighboring table in that same database called <code>schema_migrations</code>.To run the migration and create the table, you'd run <code>bin/rails db:migrate</code>, and to roll it back and delete the table, <code>bin/rails db:rollback</code>.You can learn more about migrations in the <a href="active_record_migrations.html">Active Record Migrations guide</a>.<h2 id="associations"><a class="anchorlink" href="#associations" data-turbo="false">9 Associations</a></h2>Active Record associations allow you to define relationships between models. Associations can be used to describe one-to-one, one-to-many, and many-to-many relationships. For example, a relationship like “Author has many Books” can be defined as follows:<div class="interstitial code"> <pre><code class="highlight ruby">class Author < ApplicationRecord has_many :books end </code></pre> <button class="clipboard-button" data-clipboard-text="class Author < ApplicationRecord has_many :books end ">Copy</button> </div> The <code>Author</code> class now has methods to add and remove books to an author, and much more.You can learn more about associations in the <a href="association_basics.html">Active Record Associations guide</a>. </section> <hr> <footer aria-labelledby="heading-feedback" role="region"> <h2 id="heading-feedback">Feedback</h2> You're encouraged to help improve the quality of this guide. Please contribute if you see any typos or factual errors. 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