rails--rails/guides/source/active_model_basics.md

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Active Model Basics
===================

This guide should provide you with all you need to get started using model
classes. Active Model allows for Action Pack helpers to interact with
plain Ruby objects. Active Model also helps build custom ORMs for use
outside of the Rails framework.

After reading this guide, you will know:

* How an Active Record model behaves.
* How Callbacks and validations work.
* How serializers work.
* How Active Model integrates with the Rails internationalization (i18n) framework.

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

Introduction
------------

Active Model is a library containing various modules used in developing
classes that need some features present on Active Record.
Some of these modules are explained below.

### Attribute Methods

The `ActiveModel::AttributeMethods` module can add custom prefixes and suffixes
on methods of a class. It is used by defining the prefixes and suffixes and
which methods on the object will use them.

```ruby
class Person
  include ActiveModel::AttributeMethods

  attribute_method_prefix 'reset_'
  attribute_method_suffix '_highest?'
  define_attribute_methods 'age'

  attr_accessor :age

  private
    def reset_attribute(attribute)
      send("#{attribute}=", 0)
    end

    def attribute_highest?(attribute)
      send(attribute) > 100
    end
end

person = Person.new
person.age = 110
person.age_highest?  # => true
person.reset_age     # => 0
person.age_highest?  # => false
```

### Callbacks

`ActiveModel::Callbacks` gives Active Record style callbacks. This provides an
ability to define callbacks which run at appropriate times.
After defining callbacks, you can wrap them with before, after and around
custom methods.

```ruby
class Person
  extend ActiveModel::Callbacks

  define_model_callbacks :update

  before_update :reset_me

  def update
    run_callbacks(:update) do
      # This method is called when update is called on an object.
    end
  end

  def reset_me
    # This method is called when update is called on an object as a before_update callback is defined.
  end
end
```

### Conversion

If a class defines `persisted?` and `id` methods, then you can include the
`ActiveModel::Conversion` module in that class, and call the Rails conversion
methods on objects of that class.

```ruby
class Person
  include ActiveModel::Conversion

  def persisted?
    false
  end

  def id
    nil
  end
end

person = Person.new
person.to_model == person  # => true
person.to_key              # => nil
person.to_param            # => nil
```

### Dirty

An object becomes dirty when it has gone through one or more changes to its
attributes and has not been saved. `ActiveModel::Dirty` gives the ability to
check whether an object has been changed or not. It also has attribute based
accessor methods. Let's consider a Person class with attributes `first_name`
and `last_name`:

```ruby
class Person
  include ActiveModel::Dirty
  define_attribute_methods :first_name, :last_name

  def first_name
    @first_name
  end

  def first_name=(value)
    first_name_will_change!
    @first_name = value
  end

  def last_name
    @last_name
  end

  def last_name=(value)
    last_name_will_change!
    @last_name = value
  end

  def save
    # do save work...
    changes_applied
  end
end
```

#### Querying object directly for its list of all changed attributes.

```ruby
person = Person.new
person.changed? # => false

person.first_name = "First Name"
person.first_name # => "First Name"

# returns true if any of the attributes have unsaved changes.
person.changed? # => true

# returns a list of attributes that have changed before saving.
person.changed # => ["first_name"]

# returns a Hash of the attributes that have changed with their original values.
person.changed_attributes # => {"first_name"=>nil}

# returns a Hash of changes, with the attribute names as the keys, and the
# values as an array of the old and new values for that field.
person.changes # => {"first_name"=>[nil, "First Name"]}
```

#### Attribute based accessor methods

Track whether the particular attribute has been changed or not.

```ruby
# attr_name_changed?
person.first_name # => "First Name"
person.first_name_changed? # => true
```

Track the previous value of the attribute.

```ruby
# attr_name_was accessor
person.first_name_was # => nil
```

Track both previous and current value of the changed attribute. Returns an array
if changed, otherwise returns nil.

```ruby
# attr_name_change
person.first_name_change # => [nil, "First Name"]
person.last_name_change # => nil
```

### Validations

The `ActiveModel::Validations` module adds the ability to validate objects
like in Active Record.

```ruby
class Person
  include ActiveModel::Validations

  attr_accessor :name, :email, :token

  validates :name, presence: true
  validates_format_of :email, with: /\A([^\s]+)((?:[-a-z0-9]\.)[a-z]{2,})\z/i
  validates! :token, presence: true
end

person = Person.new
person.token = "2b1f325"
person.valid?                        # => false
person.name = 'vishnu'
person.email = 'me'
person.valid?                        # => false
person.email = 'me@vishnuatrai.com'
person.valid?                        # => true
person.token = nil
person.valid?                        # => raises ActiveModel::StrictValidationFailed
```

### Naming

`ActiveModel::Naming` adds a number of class methods which make naming and routing
easier to manage. The module defines the `model_name` class method which
will define a number of accessors using some `ActiveSupport::Inflector` methods.

```ruby
class Person
  extend ActiveModel::Naming
end

Person.model_name.name                # => "Person"
Person.model_name.singular            # => "person"
Person.model_name.plural              # => "people"
Person.model_name.element             # => "person"
Person.model_name.human               # => "Person"
Person.model_name.collection          # => "people"
Person.model_name.param_key           # => "person"
Person.model_name.i18n_key            # => :person
Person.model_name.route_key           # => "people"
Person.model_name.singular_route_key  # => "person"
```

### Model

`ActiveModel::Model` adds the ability for a class to work with Action Pack and
Action View right out of the box.

```ruby
class EmailContact
  include ActiveModel::Model

  attr_accessor :name, :email, :message
  validates :name, :email, :message, presence: true

  def deliver
    if valid?
      # deliver email
    end
  end
end
```

When including `ActiveModel::Model` you get some features like:

- model name introspection
- conversions
- translations
- validations

It also gives you the ability to initialize an object with a hash of attributes,
much like any Active Record object.

```ruby
email_contact = EmailContact.new(name: 'David',
                                 email: 'david@example.com',
                                 message: 'Hello World')
email_contact.name       # => 'David'
email_contact.email      # => 'david@example.com'
email_contact.valid?     # => true
email_contact.persisted? # => false
```

Any class that includes `ActiveModel::Model` can be used with `form_for`,
`render` and any other Action View helper methods, just like Active Record
objects.

### Serialization

`ActiveModel::Serialization` provides basic serialization for your object.
You need to declare an attributes Hash which contains the attributes you want to
serialize. Attributes must be strings, not symbols.

```ruby
class Person
  include ActiveModel::Serialization

  attr_accessor :name

  def attributes
    {'name' => nil}
  end
end
```

Now you can access a serialized Hash of your object using the `serializable_hash` method.

```ruby
person = Person.new
person.serializable_hash   # => {"name"=>nil}
person.name = "Bob"
person.serializable_hash   # => {"name"=>"Bob"}
```

#### ActiveModel::Serializers

Active Model also provides the `ActiveModel::Serializers::JSON` module
for JSON serializing / deserializing. This module automatically includes the
previously discussed `ActiveModel::Serialization` module.

##### ActiveModel::Serializers::JSON

To use `ActiveModel::Serializers::JSON` you only need to change the
module you are including from `ActiveModel::Serialization` to `ActiveModel::Serializers::JSON`.

```ruby
class Person
  include ActiveModel::Serializers::JSON

  attr_accessor :name

  def attributes
    {'name' => nil}
  end
end
```

The `as_json` method, similar to `serializable_hash`, provides a Hash representing
the model.

```ruby
person = Person.new
person.as_json # => {"name"=>nil}
person.name = "Bob"
person.as_json # => {"name"=>"Bob"}
```

You can also define the attributes for a model from a JSON string.
However, you need to define the `attributes=` method on your class:

```ruby
class Person
  include ActiveModel::Serializers::JSON

  attr_accessor :name

  def attributes=(hash)
    hash.each do |key, value|
      send("#{key}=", value)
    end
  end

  def attributes
    {'name' => nil}
  end
end
```

Now it is possible to create an instance of `Person` and set attributes using `from_json`.

```ruby
json = { name: 'Bob' }.to_json
person = Person.new
person.from_json(json) # => #<Person:0x00000100c773f0 @name="Bob">
person.name            # => "Bob"
```

### Translation

`ActiveModel::Translation` provides integration between your object and the Rails
internationalization (i18n) framework.

```ruby
class Person
  extend ActiveModel::Translation
end
```

With the `human_attribute_name` method, you can transform attribute names into a
more human-readable format. The human-readable format is defined in your locale file(s).

* config/locales/app.pt-BR.yml

  ```yml
  pt-BR:
    activemodel:
      attributes:
        person:
          name: 'Nome'
  ```

```ruby
Person.human_attribute_name('name') # => "Nome"
```

### Lint Tests

`ActiveModel::Lint::Tests` allows you to test whether an object is compliant with
the Active Model API.

* `app/models/person.rb`

    ```ruby
    class Person
      include ActiveModel::Model
    end
    ```

* `test/models/person_test.rb`

    ```ruby
    require 'test_helper'

    class PersonTest < ActiveSupport::TestCase
      include ActiveModel::Lint::Tests

      setup do
        @model = Person.new
      end
    end
    ```

```bash
$ rails test

Run options: --seed 14596

# Running:

......

Finished in 0.024899s, 240.9735 runs/s, 1204.8677 assertions/s.

6 runs, 30 assertions, 0 failures, 0 errors, 0 skips
```

An object is not required to implement all APIs in order to work with
Action Pack. This module only intends to provide guidance in case you want all
features out of the box.

### SecurePassword

`ActiveModel::SecurePassword` provides a way to securely store any
password in an encrypted form. When you include this module, a
`has_secure_password` class method is provided which defines
a `password` accessor with certain validations on it.

#### Requirements

`ActiveModel::SecurePassword` depends on [`bcrypt`](https://github.com/codahale/bcrypt-ruby 'BCrypt'),
so include this gem in your `Gemfile` to use `ActiveModel::SecurePassword` correctly.
In order to make this work, the model must have an accessor named `password_digest`.
The `has_secure_password` will add the following validations on the `password` accessor:

1. Password should be present.
2. Password should be equal to its confirmation (provided `password_confirmation` is passed along).
3. The maximum length of a password is 72 (required by `bcrypt` on which ActiveModel::SecurePassword depends)

#### Examples

```ruby
class Person
  include ActiveModel::SecurePassword
  has_secure_password
  attr_accessor :password_digest
end

person = Person.new

# When password is blank.
person.valid? # => false

# When the confirmation doesn't match the password.
person.password = 'aditya'
person.password_confirmation = 'nomatch'
person.valid? # => false

# When the length of password exceeds 72.
person.password = person.password_confirmation = 'a' * 100
person.valid? # => false

# When only password is supplied with no password_confirmation.
person.password = 'aditya'
person.valid? # => true

# When all validations are passed.
person.password = person.password_confirmation = 'aditya'
person.valid? # => true
```