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](getting_started.html#the-mvc-architecture) - 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 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 is 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, 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.

### 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 case 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 times then this 
should be the default way. In this cases, explicit configuration would be needed 
only in those cases where you can't follow the conventions for any reason.

### 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 to pluralize (and 
singularize) 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:

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

| Model / Class | Table / Schema |
| ------------- | -------------- |
| `Post`        | `posts`        |
| `LineItem`    | `line_items`   |
| `Deer`        | `deer`         |
| `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. When using [Rails 
  Migrations](migrations.html) to create your tables, this column will be 
  automatically created.

There are also some optional column names that will create 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 updated.
* `lock_version` - Adds [optimistic 
  locking](http://api.rubyonrails.org/classes/ActiveRecord/Locking.html) to 
  a model.
* `type` - Specifies that the model uses [Single Table 
  Inheritance](http://api.rubyonrails.org/classes/ActiveRecord/Base.html)
* `(table_name)_count` - Used to cache the number of belonging objects on 
  associations. For example, a `comments_count` column in a `Post` class that 
  has many instances of `Comment` will cache the number of existent comments 
  for each post.

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 `ActiveRecord::Base` class and you're good to go:

```ruby
class Product < ActiveRecord::Base
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 sentence like:

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

Following the table schema above, 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.

You can use the `ActiveRecord::Base.table_name=` method to specify the table 
name that should be used:

```ruby
class Product < ActiveRecord::Base
  self.table_name = "PRODUCT"
end
```

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

```ruby
class FunnyJoke < ActiveSupport::TestCase
  set_fixture_class funny_jokes: 'Joke'
  fixtures :funny_jokes
  ...
end
```

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

```ruby
class Product < ActiveRecord::Base
  set_primary_key "product_id"
end
```

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
```

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 database, so 
the methods `create`, `save` and `update` take it into account when 
running: they return `false` when validation fails and they didn't actually 
perform any operation on database. All of these have a bang counterpart (that 
is, `create!`, `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 < ActiveRecord::Base
  validates_presence_of :name
end

User.create  # => false
User.create! # => 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 support using `rake`. Here's a migration that 
creates a table:

```ruby
class CreatePublications < ActiveRecord::Migration
  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 `rake db:migrate`
and to roll it back, `rake 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](migrations.html)
