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Active Record Basics
====================

This guide is an introduction to Active Record.

After reading this guide, you will know:

* What Object Relational Mapping and Active Record are and how they are used in
  Rails.
* How Active Record fits into the Model-View-Controller paradigm.
* How to use Active Record models to manipulate data stored in a relational
  database.
* Active Record schema naming conventions.
* The concepts of database migrations, validations, and callbacks.

--------------------------------------------------------------------------------

What is Active Record?
----------------------

Active Record is the M in [MVC](https://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller) - the
model - which is the layer of the system responsible for representing business
data and logic. Active Record facilitates the creation and use of business
objects whose data requires persistent storage to a database. It is an
implementation of the Active Record pattern which itself is a description of an
Object Relational Mapping system.

### The Active Record Pattern

[Active Record was described by Martin Fowler](https://www.martinfowler.com/eaaCatalog/activeRecord.html)
in his book _Patterns of Enterprise Application Architecture_. In
Active Record, objects carry both persistent data and behavior which
operates on that data. Active Record takes the opinion that ensuring
data access logic as part of the object will educate users of that
object on how to write to and read from the database.

### Object Relational Mapping

[Object Relational Mapping](https://en.wikipedia.org/wiki/Object-relational_mapping), commonly referred to as its abbreviation ORM, is
a technique that connects the rich objects of an application to tables in
a relational database management system. Using ORM, the properties and
relationships of the objects in an application can be easily stored and
retrieved from a database without writing SQL statements directly and with less
overall database access code.

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 [this tutorial](https://www.w3schools.com/sql/default.asp) (or [this one](http://www.sqlcourse.com/)) or study them by other means if you would like to learn more.

### Active Record as an ORM Framework

Active Record gives us several mechanisms, the most important being the ability
to:

* Represent models and their data.
* Represent associations between these models.
* Represent inheritance hierarchies through related models.
* Validate models before they get persisted to the database.
* Perform database operations in an object-oriented fashion.

Convention over Configuration in Active Record
----------------------------------------------

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 need to write very little configuration (in some cases no
configuration at all) when creating Active Record models. The idea is that if
you configure your applications in the very same way most of the time then this
should be the default way. Thus, explicit configuration would be needed
only in those cases where you can't follow the standard convention.

### Naming Conventions

By default, Active Record uses some naming conventions to find out how the
mapping between models and database tables should be created. Rails will
pluralize your class names to find the respective database table. So, for
a class `Book`, you should have a database table called **books**. The Rails
pluralization mechanisms are very powerful, being capable of pluralizing (and
singularizing) both regular and irregular words. When using class names composed
of two or more words, the model class name should follow the Ruby conventions,
using the CamelCase form, while the table name must contain the words separated
by underscores. Examples:

* Model Class - Singular with the first letter of each word capitalized (e.g.,
`BookClub`).
* Database Table - Plural with underscores separating words (e.g., `book_clubs`).

| Model / Class    | Table / Schema |
| ---------------- | -------------- |
| `Article`        | `articles`     |
| `LineItem`       | `line_items`   |
| `Deer`           | `deers`        |
| `Mouse`          | `mice`         |
| `Person`         | `people`       |


### Schema Conventions

Active Record uses naming conventions for the columns in database tables,
depending on the purpose of these columns.

* **Foreign keys** - These fields should be named following the pattern
  `singularized_table_name_id` (e.g., `item_id`, `order_id`). These are the
  fields that Active Record will look for when you create associations between
  your models.
* **Primary keys** - By default, Active Record will use an integer column named
  `id` as the table's primary key (`bigint` for PostgreSQL and MySQL, `integer`
  for SQLite). When using [Active Record Migrations](active_record_migrations.html)
  to create your tables, this column will be automatically created.

There are also some optional column names that will add additional features
to Active Record instances:

* `created_at` - Automatically gets set to the current date and time when the
  record is first created.
* `updated_at` - Automatically gets set to the current date and time whenever
  the record is created or updated.
* `lock_version` - Adds [optimistic
  locking](https://api.rubyonrails.org/classes/ActiveRecord/Locking.html) to
  a model.
* `type` - Specifies that the model uses [Single Table
  Inheritance](https://api.rubyonrails.org/classes/ActiveRecord/Base.html#class-ActiveRecord::Base-label-Single+table+inheritance).
* `(association_name)_type` - Stores the type for
  [polymorphic associations](association_basics.html#polymorphic-associations).
* `(table_name)_count` - Used to cache the number of belonging objects on
  associations. For example, a `comments_count` column in an `Article` class that
  has many instances of `Comment` will cache the number of existent comments
  for each article.

NOTE: While these column names are optional, they are in fact reserved by Active Record. Steer clear of reserved keywords unless you want the extra functionality. For example, `type` is a reserved keyword used to designate a table using Single Table Inheritance (STI). If you are not using STI, try an analogous keyword like "context", that may still accurately describe the data you are modeling.

Creating Active Record Models
-----------------------------

It is very easy to create Active Record models. All you have to do is to
subclass the `ApplicationRecord` class and you're good to go:

```ruby
class Product < ApplicationRecord
end
```

This will create a `Product` model, mapped to a `products` table at the
database. By doing this you'll also have the ability to map the columns of each
row in that table with the attributes of the instances of your model. Suppose
that the `products` table was created using an SQL (or one of its extensions) statement like:

```sql
CREATE TABLE products (
   id int(11) NOT NULL auto_increment,
   name varchar(255),
   PRIMARY KEY  (id)
);
```

Schema above declares a table with two columns: `id` and `name`. Each row of
this table represents a certain product with these two parameters. Thus, you
would be able to write code like the following:

```ruby
p = Product.new
p.name = "Some Book"
puts p.name # "Some Book"
```

Overriding the Naming Conventions
---------------------------------

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.

`ApplicationRecord` inherits from `ActiveRecord::Base`, which defines a
number of helpful methods. You can use the `ActiveRecord::Base.table_name=`
method to specify the table name that should be used:

```ruby
class Product < ApplicationRecord
  self.table_name = "my_products"
end
```

If you do so, you will have to define manually the class name that is hosting
the fixtures (my_products.yml) using the `set_fixture_class` method in your test
definition:

```ruby
class ProductTest < ActiveSupport::TestCase
  set_fixture_class my_products: Product
  fixtures :my_products
  ...
end
```

It's also possible to override the column that should be used as the table's
primary key using the `ActiveRecord::Base.primary_key=` method:

```ruby
class Product < ApplicationRecord
  self.primary_key = "product_id"
end
```

NOTE: Active Record does not support using non-primary key columns named `id`.

CRUD: Reading and Writing Data
------------------------------

CRUD is an acronym for the four verbs we use to operate on data: **C**reate,
**R**ead, **U**pdate and **D**elete. Active Record automatically creates methods
to allow an application to read and manipulate data stored within its tables.

### Create

Active Record objects can be created from a hash, a block, or have their
attributes manually set after creation. The `new` method will return a new
object while `create` will return the object and save it to the database.

For example, given a model `User` with attributes of `name` and `occupation`,
the `create` method call will create and save a new record into the database:

```ruby
user = User.create(name: "David", occupation: "Code Artist")
```

Using the `new` method, an object can be instantiated without being saved:

```ruby
user = User.new
user.name = "David"
user.occupation = "Code Artist"
```

A call to `user.save` will commit the record to the database.

Finally, if a block is provided, both `create` and `new` will yield the new
object to that block for initialization:

```ruby
user = User.new do |u|
  u.name = "David"
  u.occupation = "Code Artist"
end
```

### Read

Active Record provides a rich API for accessing data within a database. Below
are a few examples of different data access methods provided by Active Record.

```ruby
# return a collection with all users
users = User.all
```

```ruby
# return the first user
user = User.first
```

```ruby
# return the first user named David
david = User.find_by(name: 'David')
```

```ruby
# find all users named David who are Code Artists and sort by created_at in reverse chronological order
users = User.where(name: 'David', occupation: 'Code Artist').order(created_at: :desc)
```

You can learn more about querying an Active Record model in the [Active Record
Query Interface](active_record_querying.html) guide.

### Update

Once an Active Record object has been retrieved, its attributes can be modified
and it can be saved to the database.

```ruby
user = User.find_by(name: 'David')
user.name = 'Dave'
user.save
```

A shorthand for this is to use a hash mapping attribute names to the desired
value, like so:

```ruby
user = User.find_by(name: 'David')
user.update(name: 'Dave')
```

This is most useful when updating several attributes at once. If, on the other
hand, you'd like to update several records in bulk, you may find the
`update_all` class method useful:

```ruby
User.update_all "max_login_attempts = 3, must_change_password = 'true'"
```

### Delete

Likewise, once retrieved an Active Record object can be destroyed which removes
it from the database.

```ruby
user = User.find_by(name: 'David')
user.destroy
```

If you'd like to delete several records in bulk, you may use `destroy_by`
or `destroy_all` method:

```ruby
# find and delete all users named David
User.destroy_by(name: 'David')

# delete all users
User.destroy_all
```

Validations
-----------

Active Record allows you to validate the state of a model before it gets written
into the database. There are several methods that you can use to check your
models and validate that an attribute value is not empty, is unique and not
already in the database, follows a specific format, and many more.

Validation is a very important issue to consider when persisting to the database, so
the methods `save` and `update` take it into account when
running: they return `false` when validation fails and they don't actually
perform any operations on the database. All of these have a bang counterpart (that
is, `save!` and `update!`), which are stricter in that
they raise the exception `ActiveRecord::RecordInvalid` if validation fails.
A quick example to illustrate:

```ruby
class User < ApplicationRecord
  validates :name, presence: true
end

user = User.new
user.save  # => false
user.save! # => ActiveRecord::RecordInvalid: Validation failed: Name can't be blank
```

You can learn more about validations in the [Active Record Validations
guide](active_record_validations.html).

Callbacks
---------

Active Record callbacks allow you to attach code to certain events in the
life-cycle of your models. This enables you to add behavior to your models by
transparently executing code when those events occur, like when you create a new
record, update it, destroy it, and so on. You can learn more about callbacks in
the [Active Record Callbacks guide](active_record_callbacks.html).

Migrations
----------

Rails provides a domain-specific language for managing a database schema called
migrations. Migrations are stored in files which are executed against any
database that Active Record supports using `rake`. Here's a migration that
creates a table:

```ruby
class CreatePublications < ActiveRecord::Migration[5.0]
  def change
    create_table :publications do |t|
      t.string :title
      t.text :description
      t.references :publication_type
      t.integer :publisher_id
      t.string :publisher_type
      t.boolean :single_issue

      t.timestamps
    end
    add_index :publications, :publication_type_id
  end
end
```

Rails keeps track of which files have been committed to the database and
provides rollback features. To actually create the table, you'd run `rails db:migrate`
and to roll it back, `rails db:rollback`.

Note that the above code is database-agnostic: it will run in MySQL,
PostgreSQL, Oracle, and others. You can learn more about migrations in the
[Active Record Migrations guide](active_record_migrations.html).