Active Record Validations
=========================

This guide teaches you how to validate the state of objects before they go into
the database using Active Record's validations feature.

After reading this guide, you will know:

* Use the built-in Active Record validation helpers
* Create your own custom validation methods
* Work with the error messages generated by the validation process

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

Validations Overview
--------------------

Here's an example of a very simple validation:

```ruby
class Person < ActiveRecord::Base
  validates :name, presence: true
end

Person.create(name: "John Doe").valid? # => true
Person.create(name: nil).valid? # => false
```

As you can see, our validation lets us know that our `Person` is not valid
without a `name` attribute. The second `Person` will not be persisted to the
database.

Before we dig into more details, let's talk about how validations fit into the
big picture of your application.

### Why Use Validations?

Validations are used to ensure that only valid data is saved into your
database. For example, it may be important to your application to ensure that
every user provides a valid email address and mailing address. Model-level
validations are the best way to ensure that only valid data is saved into your
database. They are database agnostic, cannot be bypassed by end users, and are
convenient to test and maintain. Rails makes them easy to use, provides
built-in helpers for common needs, and allows you to create your own validation
methods as well.

There are several other ways to validate data before it is saved into your
database, including native database constraints, client-side validations,
controller-level validations. Here's a summary of the pros and cons:

* Database constraints and/or stored procedures make the validation mechanisms
  database-dependent and can make testing and maintenance more difficult.
  However, if your database is used by other applications, it may be a good
  idea to use some constraints at the database level. Additionally,
  database-level validations can safely handle some things (such as uniqueness
  in heavily-used tables) that can be difficult to implement otherwise.
* Client-side validations can be useful, but are generally unreliable if used
  alone. If they are implemented using JavaScript, they may be bypassed if
  JavaScript is turned off in the user's browser. However, if combined with
  other techniques, client-side validation can be a convenient way to provide
  users with immediate feedback as they use your site.
* Controller-level validations can be tempting to use, but often become
  unwieldy and difficult to test and maintain. Whenever possible, it's a good
  idea to keep your controllers skinny, as it will make your application a
  pleasure to work with in the long run.

Choose these in certain, specific cases. It's the opinion of the Rails team
that model-level validations are the most appropriate in most circumstances.

### When Does Validation Happen?

There are two kinds of Active Record objects: those that correspond to a row
inside your database and those that do not. When you create a fresh object, for
example using the `new` method, that object does not belong to the database
yet. Once you call `save` upon that object it will be saved into the
appropriate database table. Active Record uses the `new_record?` instance
method to determine whether an object is already in the database or not.
Consider the following simple Active Record class:

```ruby
class Person < ActiveRecord::Base
end
```

We can see how it works by looking at some `rails console` output:

```ruby
$ rails console
>> p = Person.new(name: "John Doe")
=> #<Person id: nil, name: "John Doe", created_at: nil, updated_at: nil>
>> p.new_record?
=> true
>> p.save
=> true
>> p.new_record?
=> false
```

Creating and saving a new record will send an SQL `INSERT` operation to the
database. Updating an existing record will send an SQL `UPDATE` operation
instead. Validations are typically run before these commands are sent to the
database. If any validations fail, the object will be marked as invalid and
Active Record will not perform the `INSERT` or `UPDATE` operation. This avoids
storing an invalid object in the database. You can choose to have specific
validations run when an object is created, saved, or updated.

CAUTION: There are many ways to change the state of an object in the database.
Some methods will trigger validations, but some will not. This means that it's
possible to save an object in the database in an invalid state if you aren't
careful.

The following methods trigger validations, and will save the object to the
database only if the object is valid:

* `create`
* `create!`
* `save`
* `save!`
* `update`
* `update_attributes`
* `update_attributes!`

The bang versions (e.g. `save!`) raise an exception if the record is invalid.
The non-bang versions don't: `save` and `update_attributes` return `false`,
`create` and `update` just return the objects.

### Skipping Validations

The following methods skip validations, and will save the object to the
database regardless of its validity. They should be used with caution.

* `decrement!`
* `decrement_counter`
* `increment!`
* `increment_counter`
* `toggle!`
* `touch`
* `update_all`
* `update_attribute`
* `update_column`
* `update_columns`
* `update_counters`

Note that `save` also has the ability to skip validations if passed `validate:
false` as argument. This technique should be used with caution.

* `save(validate: false)`

### `valid?` and `invalid?`

To verify whether or not an object is valid, Rails uses the `valid?` method.
You can also use this method on your own. `valid?` triggers your validations
and returns true if no errors were found in the object, and false otherwise.
As you saw above:

```ruby
class Person < ActiveRecord::Base
  validates :name, presence: true
end

Person.create(name: "John Doe").valid? # => true
Person.create(name: nil).valid? # => false
```

After Active Record has performed validations, any errors found can be accessed
through the `errors` instance method, which returns a collection of errors. By
definition, an object is valid if this collection is empty after running
validations.

Note that an object instantiated with `new` will not report errors even if it's
technically invalid, because validations are not run when using `new`.

```ruby
class Person < ActiveRecord::Base
  validates :name, presence: true
end

>> p = Person.new
#=> #<Person id: nil, name: nil>
>> p.errors
#=> {}

>> p.valid?
#=> false
>> p.errors
#=> {name:["can't be blank"]}

>> p = Person.create
#=> #<Person id: nil, name: nil>
>> p.errors
#=> {name:["can't be blank"]}

>> p.save
#=> false

>> p.save!
#=> ActiveRecord::RecordInvalid: Validation failed: Name can't be blank

>> Person.create!
#=> ActiveRecord::RecordInvalid: Validation failed: Name can't be blank
```

`invalid?` is simply the inverse of `valid?`. It triggers your validations,
returning true if any errors were found in the object, and false otherwise.

### `errors[]`

To verify whether or not a particular attribute of an object is valid, you can
use `errors[:attribute]`. It returns an array of all the errors for
`:attribute`. If there are no errors on the specified attribute, an empty array
is returned.

This method is only useful _after_ validations have been run, because it only
inspects the errors collection and does not trigger validations itself. It's
different from the `ActiveRecord::Base#invalid?` method explained above because
it doesn't verify the validity of the object as a whole. It only checks to see
whether there are errors found on an individual attribute of the object.

```ruby
class Person < ActiveRecord::Base
  validates :name, presence: true
end

>> Person.new.errors[:name].any? # => false
>> Person.create.errors[:name].any? # => true
```

We'll cover validation errors in greater depth in the [Working with Validation
Errors](#working-with-validation-errors) section. For now, let's turn to the
built-in validation helpers that Rails provides by default.

Validation Helpers
------------------

Active Record offers many pre-defined validation helpers that you can use
directly inside your class definitions. These helpers provide common validation
rules. Every time a validation fails, an error message is added to the object's
`errors` collection, and this message is associated with the attribute being
validated.

Each helper accepts an arbitrary number of attribute names, so with a single
line of code you can add the same kind of validation to several attributes.

All of them accept the `:on` and `:message` options, which define when the
validation should be run and what message should be added to the `errors`
collection if it fails, respectively. The `:on` option takes one of the values
`:save` (the default), `:create`  or `:update`. There is a default error
message for each one of the validation helpers. These messages are used when
the `:message` option isn't specified. Let's take a look at each one of the
available helpers.

### `acceptance`

This method validates that a checkbox on the user interface was checked when a
form was submitted. This is typically used when the user needs to agree to your
application's terms of service, confirm reading some text, or any similar
concept. This validation is very specific to web applications and this
'acceptance' does not need to be recorded anywhere in your database (if you
don't have a field for it, the helper will just create a virtual attribute).

```ruby
class Person < ActiveRecord::Base
  validates :terms_of_service, acceptance: true
end
```

The default error message for this helper is _"must be accepted"_.

It can receive an `:accept` option, which determines the value that will be
considered acceptance. It defaults to "1" and can be easily changed.

```ruby
class Person < ActiveRecord::Base
  validates :terms_of_service, acceptance: { accept: 'yes' }
end
```

### `validates_associated`

You should use this helper when your model has associations with other models
and they also need to be validated. When you try to save your object, `valid?`
will be called upon each one of the associated objects.

```ruby
class Library < ActiveRecord::Base
  has_many :books
  validates_associated :books
end
```

This validation will work with all of the association types.

CAUTION: Don't use `validates_associated` on both ends of your associations.
They would call each other in an infinite loop.

The default error message for `validates_associated` is _"is invalid"_. Note
that each associated object will contain its own `errors` collection; errors do
not bubble up to the calling model.

### `confirmation`

You should use this helper when you have two text fields that should receive
exactly the same content. For example, you may want to confirm an email address
or a password. This validation creates a virtual attribute whose name is the
name of the field that has to be confirmed with "_confirmation" appended.

```ruby
class Person < ActiveRecord::Base
  validates :email, confirmation: true
end
```

In your view template you could use something like

```erb
<%= text_field :person, :email %>
<%= text_field :person, :email_confirmation %>
```

This check is performed only if `email_confirmation` is not `nil`. To require
confirmation, make sure to add a presence check for the confirmation attribute
(we'll take a look at `presence` later on this guide):

```ruby
class Person < ActiveRecord::Base
  validates :email, confirmation: true
  validates :email_confirmation, presence: true
end
```

The default error message for this helper is _"doesn't match confirmation"_.

### `exclusion`

This helper validates that the attributes' values are not included in a given
set. In fact, this set can be any enumerable object.

```ruby
class Account < ActiveRecord::Base
  validates :subdomain, exclusion: { in: %w(www us ca jp),
    message: "Subdomain %{value} is reserved." }
end
```

The `exclusion` helper has an option `:in` that receives the set of values that
will not be accepted for the validated attributes. The `:in` option has an
alias called `:within` that you can use for the same purpose, if you'd like to.
This example uses the `:message` option to show how you can include the
attribute's value.

The default error message is _"is reserved"_.

### `format`

This helper validates the attributes' values by testing whether they match a
given regular expression, which is specified using the `:with` option.

```ruby
class Product < ActiveRecord::Base
  validates :legacy_code, format: { with: /\A[a-zA-Z]+\z/,
    message: "Only letters allowed" }
end
```

The default error message is _"is invalid"_.

### `inclusion`

This helper validates that the attributes' values are included in a given set.
In fact, this set can be any enumerable object.

```ruby
class Coffee < ActiveRecord::Base
  validates :size, inclusion: { in: %w(small medium large),
    message: "%{value} is not a valid size" }
end
```

The `inclusion` helper has an option `:in` that receives the set of values that
will be accepted. The `:in` option has an alias called `:within` that you can
use for the same purpose, if you'd like to. The previous example uses the
`:message` option to show how you can include the attribute's value.

The default error message for this helper is _"is not included in the list"_.

### `length`

This helper validates the length of the attributes' values. It provides a
variety of options, so you can specify length constraints in different ways:

```ruby
class Person < ActiveRecord::Base
  validates :name, length: { minimum: 2 }
  validates :bio, length: { maximum: 500 }
  validates :password, length: { in: 6..20 }
  validates :registration_number, length: { is: 6 }
end
```

The possible length constraint options are:

* `:minimum` - The attribute cannot have less than the specified length.
* `:maximum` - The attribute cannot have more than the specified length.
* `:in` (or `:within`) - The attribute length must be included in a given
  interval. The value for this option must be a range.
* `:is` - The attribute length must be equal to the given value.

The default error messages depend on the type of length validation being
performed. You can personalize these messages using the `:wrong_length`,
`:too_long`, and `:too_short` options and `%{count}` as a placeholder for the
number corresponding to the length constraint being used. You can still use the
`:message` option to specify an error message.

```ruby
class Person < ActiveRecord::Base
  validates :bio, length: { maximum: 1000,
    too_long: "%{count} characters is the maximum allowed" }
end
```

This helper counts characters by default, but you can split the value in a
different way using the `:tokenizer` option:

```ruby
class Essay < ActiveRecord::Base
  validates :content, length: {
    minimum: 300,
    maximum: 400,
    tokenizer: lambda { |str| str.scan(/\w+/) },
    too_short: "must have at least %{count} words",
    too_long: "must have at most %{count} words"
  }
end
```

Note that the default error messages are plural (e.g., "is too short (minimum
is %{count} characters)"). For this reason, when `:minimum` is 1 you should
provide a personalized message or use `validates_presence_of` instead. When
`:in` or `:within` have a lower limit of 1, you should either provide a
personalized message or call `presence` prior to `length`.

The `size` helper is an alias for `length`.

### `numericality`

This helper validates that your attributes have only numeric values. By
default, it will match an optional sign followed by an integral or floating
point number. To specify that only integral numbers are allowed set
`:only_integer` to true.

If you set `:only_integer` to `true`, then it will use the

```ruby
/\A[+-]?\d+\Z/
```

regular expression to validate the attribute's value. Otherwise, it will try to
convert the value to a number using `Float`.

WARNING. Note that the regular expression above allows a trailing newline
character.

```ruby
class Player < ActiveRecord::Base
  validates :points, numericality: true
  validates :games_played, numericality: { only_integer: true }
end
```

Besides `:only_integer`, this helper also accepts the following options to add
constraints to acceptable values:

* `:greater_than` - Specifies the value must be greater than the supplied
  value. The default error message for this option is _"must be greater than
  %{count}"_.
* `:greater_than_or_equal_to` - Specifies the value must be greater than or
  equal to the supplied value. The default error message for this option is
  _"must be greater than or equal to %{count}"_.
* `:equal_to` - Specifies the value must be equal to the supplied value. The
  default error message for this option is _"must be equal to %{count}"_.
* `:less_than` - Specifies the value must be less than the supplied value. The
  default error message for this option is _"must be less than %{count}"_.
* `:less_than_or_equal_to` - Specifies the value must be less than or equal the
  supplied value. The default error message for this option is _"must be less
  than or equal to %{count}"_.
* `:odd` - Specifies the value must be an odd number if set to true. The
  default error message for this option is _"must be odd"_.
* `:even` - Specifies the value must be an even number if set to true. The
  default error message for this option is _"must be even"_.

The default error message is _"is not a number"_.

### `presence`

This helper validates that the specified attributes are not empty. It uses the
`blank?` method to check if the value is either `nil` or a blank string, that
is, a string that is either empty or consists of whitespace.

```ruby
class Person < ActiveRecord::Base
  validates :name, :login, :email, presence: true
end
```

If you want to be sure that an association is present, you'll need to test
whether the foreign key used to map the association is present, and not the
associated object itself.

```ruby
class LineItem < ActiveRecord::Base
  belongs_to :order
  validates :order, presence: true
end
```

You should also be sure to have a proper `:inverse_of` as well:

```ruby
class Order < ActiveRecord::Base
  has_many :line_items, inverse_of: :order
end
```

If you validate the presence of an object associated via a `has_one` or
`has_many` relationship, it will check that the object is neither `blank?` nor
`marked_for_destruction?`.

Since `false.blank?` is true, if you want to validate the presence of a boolean
field you should use `validates :field_name, inclusion: { in: [true, false] }`.

The default error message is _"can't be empty"_.

### `uniqueness`

This helper validates that the attribute's value is unique right before the
object gets saved. It does not create a uniqueness constraint in the database,
so it may happen that two different database connections create two records
with the same value for a column that you intend to be unique. To avoid that,
you must create a unique index in your database.

```ruby
class Account < ActiveRecord::Base
  validates :email, uniqueness: true
end
```

The validation happens by performing an SQL query into the model's table,
searching for an existing record with the same value in that attribute.

There is a `:scope` option that you can use to specify other attributes that
are used to limit the uniqueness check:

```ruby
class Holiday < ActiveRecord::Base
  validates :name, uniqueness: { scope: :year,
    message: "should happen once per year" }
end
```

There is also a `:case_sensitive` option that you can use to define whether the
uniqueness constraint will be case sensitive or not. This option defaults to
true.

```ruby
class Person < ActiveRecord::Base
  validates :name, uniqueness: { case_sensitive: false }
end
```

WARNING. Note that some databases are configured to perform case-insensitive
searches anyway.

The default error message is _"has already been taken"_.

### `validates_with`

This helper passes the record to a separate class for validation.

```ruby
class Person < ActiveRecord::Base
  validates_with GoodnessValidator
end

class GoodnessValidator < ActiveModel::Validator
  def validate(record)
    if record.first_name == "Evil"
      record.errors[:base] << "This person is evil"
    end
  end
end
```

NOTE: Errors added to `record.errors[:base]` relate to the state of the record
as a whole, and not to a specific attribute.

The `validates_with` helper takes a class, or a list of classes to use for
validation. There is no default error message for `validates_with`. You must
manually add errors to the record's errors collection in the validator class.

To implement the validate method, you must have a `record` parameter defined,
which is the record to be validated.

Like all other validations, `validates_with` takes the `:if`, `:unless` and
`:on` options. If you pass any other options, it will send those options to the
validator class as `options`:

```ruby
class Person < ActiveRecord::Base
  validates_with GoodnessValidator, fields: [:first_name, :last_name]
end

class GoodnessValidator < ActiveModel::Validator
  def validate(record)
    if options[:fields].any?{|field| record.send(field) == "Evil" }
      record.errors[:base] << "This person is evil"
    end
  end
end
```

### `validates_each`

This helper validates attributes against a block. It doesn't have a predefined
validation function. You should create one using a block, and every attribute
passed to `validates_each` will be tested against it. In the following example,
we don't want names and surnames to begin with lower case.

```ruby
class Person < ActiveRecord::Base
  validates_each :name, :surname do |record, attr, value|
    record.errors.add(attr, 'must start with upper case') if value =~ /\A[a-z]/
  end
end
```

The block receives the record, the attribute's name and the attribute's value.
You can do anything you like to check for valid data within the block. If your
validation fails, you should add an error message to the model, therefore
making it invalid.

Common Validation Options
-------------------------

These are common validation options:

### `:allow_nil`

The `:allow_nil` option skips the validation when the value being validated is
`nil`.

```ruby
class Coffee < ActiveRecord::Base
  validates :size, inclusion: { in: %w(small medium large),
    message: "%{value} is not a valid size" }, allow_nil: true
end
```

TIP: `:allow_nil` is ignored by the presence validator.

### `:allow_blank`

The `:allow_blank` option is similar to the `:allow_nil` option. This option
will let validation pass if the attribute's value is `blank?`, like `nil` or an
empty string for example.

```ruby
class Topic < ActiveRecord::Base
  validates :title, length: { is: 5 }, allow_blank: true
end

Topic.create("title" => "").valid?  # => true
Topic.create("title" => nil).valid? # => true
```

TIP: `:allow_blank` is ignored by the presence validator.

### `:message`

As you've already seen, the `:message` option lets you specify the message that
will be added to the `errors` collection when validation fails. When this
option is not used, Active Record will use the respective default error message
for each validation helper.

### `:on`

The `:on` option lets you specify when the validation should happen. The
default behavior for all the built-in validation helpers is to be run on save
(both when you're creating a new record and when you're updating it). If you
want to change it, you can use `on: :create` to run the validation only when a
new record is created or `on: :update` to run the validation only when a record
is updated.

```ruby
class Person < ActiveRecord::Base
  # it will be possible to update email with a duplicated value
  validates :email, uniqueness: true, on: :create

  # it will be possible to create the record with a non-numerical age
  validates :age, numericality: true, on: :update

  # the default (validates on both create and update)
  validates :name, presence: true, on: :save
end
```

Strict Validations
------------------

You can also specify validations to be strict and raise
`ActiveModel::StrictValidationFailed` when the object is invalid.

```ruby
class Person < ActiveRecord::Base
  validates :name, presence: { strict: true }
end

Person.new.valid?  # => ActiveModel::StrictValidationFailed: Name can't be blank
```

There is also an ability to pass custom exception to `:strict` option

```ruby
class Person < ActiveRecord::Base
  validates :token, presence: true, uniqueness: true, strict: TokenGenerationException
end

Person.new.valid?  # => TokenGenerationException: Token can't be blank
```

Conditional Validation
----------------------

Sometimes it will make sense to validate an object only when a given predicate
is satisfied. You can do that by using the `:if` and `:unless` options, which
can take a symbol, a string, a `Proc` or an `Array`. You may use the `:if`
option when you want to specify when the validation **should** happen. If you
want to specify when the validation **should not** happen, then you may use the
`:unless` option.

### Using a Symbol with `:if` and `:unless`

You can associate the `:if` and `:unless` options with a symbol corresponding
to the name of a method that will get called right before validation happens.
This is the most commonly used option.

```ruby
class Order < ActiveRecord::Base
  validates :card_number, presence: true, if: :paid_with_card?

  def paid_with_card?
    payment_type == "card"
  end
end
```

### Using a String with `:if` and `:unless`

You can also use a string that will be evaluated using `eval` and needs to
contain valid Ruby code. You should use this option only when the string
represents a really short condition.

```ruby
class Person < ActiveRecord::Base
  validates :surname, presence: true, if: "name.nil?"
end
```

### Using a Proc with `:if` and `:unless`

Finally, it's possible to associate `:if` and `:unless` with a `Proc` object
which will be called. Using a `Proc` object gives you the ability to write an
inline condition instead of a separate method. This option is best suited for
one-liners.

```ruby
class Account < ActiveRecord::Base
  validates :password, confirmation: true,
    unless: Proc.new { |a| a.password.blank? }
end
```

### Grouping conditional validations

Sometimes it is useful to have multiple validations use one condition, it can
be easily achieved using `with_options`.

```ruby
class User < ActiveRecord::Base
  with_options if: :is_admin? do |admin|
    admin.validates :password, length: { minimum: 10 }
    admin.validates :email, presence: true
  end
end
```

All validations inside of `with_options` block will have automatically passed
the condition `if: :is_admin?`

### Combining validation conditions

On the other hand, when multiple conditions define whether or not a validation
should happen, an `Array` can be used. Moreover, you can apply both `:if` and
`:unless` to the same validation.

```ruby
class Computer < ActiveRecord::Base
  validates :mouse, presence: true,
                    if: ["market.retail?", :desktop?]
                    unless: Proc.new { |c| c.trackpad.present? }
end
```

The validation only runs when all the `:if` conditions and none of the
`:unless` conditions are evaluated to `true`.

Performing Custom Validations
-----------------------------

When the built-in validation helpers are not enough for your needs, you can
write your own validators or validation methods as you prefer.

### Custom Validators

Custom validators are classes that extend `ActiveModel::Validator`. These
classes must implement a `validate` method which takes a record as an argument
and performs the validation on it. The custom validator is called using the
`validates_with` method.

```ruby
class MyValidator < ActiveModel::Validator
  def validate(record)
    unless record.name.starts_with? 'X'
      record.errors[:name] << 'Need a name starting with X please!'
    end
  end
end

class Person
  include ActiveModel::Validations
  validates_with MyValidator
end
```

The easiest way to add custom validators for validating individual attributes
is with the convenient `ActiveModel::EachValidator`. In this case, the custom
validator class must implement a `validate_each` method which takes three
arguments: record, attribute and value which correspond to the instance, the
attribute to be validated and the value of the attribute in the passed
instance.

```ruby
class EmailValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless value =~ /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i
      record.errors[attribute] << (options[:message] || "is not an email")
    end
  end
end

class Person < ActiveRecord::Base
  validates :email, presence: true, email: true
end
```

As shown in the example, you can also combine standard validations with your
own custom validators.

### Custom Methods

You can also create methods that verify the state of your models and add
messages to the `errors` collection when they are invalid. You must then
register these methods by using the `validate` class method, passing in the
symbols for the validation methods' names.

You can pass more than one symbol for each class method and the respective
validations will be run in the same order as they were registered.

```ruby
class Invoice < ActiveRecord::Base
  validate :expiration_date_cannot_be_in_the_past,
    :discount_cannot_be_greater_than_total_value

  def expiration_date_cannot_be_in_the_past
    if expiration_date.present? && expiration_date < Date.today
      errors.add(:expiration_date, "can't be in the past")
    end
  end

  def discount_cannot_be_greater_than_total_value
    if discount > total_value
      errors.add(:discount, "can't be greater than total value")
    end
  end
end
```

By default such validations will run every time you call `valid?`. It is also
possible to control when to run these custom validations by giving an `:on`
option to the `validate` method, with either: `:create` or `:update`.

```ruby
class Invoice < ActiveRecord::Base
  validate :active_customer, on: :create

  def active_customer
    errors.add(:customer_id, "is not active") unless customer.active?
  end
end
```

Displaying Validation Errors in Views
-------------------------------------

Once you've created a model and added validations, if that model is created via
a web form, you probably want to display an error message when one of the
validations fail.

Because every application handles this kind of thing differently, Rails does
not include any view helpers to help you generate these messages directly.
However, due to the rich number of methods Rails gives you to interact with
validations in general, it's fairly easy to build your own. In addition, when
generating a scaffold, Rails will put some ERB into the `_form.html.erb` that
it generates that displays the full list of errors on that model.

Assuming we have a model that's been saved in an instance variable named
`@post`, it looks like this:

```ruby
<% if @post.errors.any? %>
  <div id="error_explanation">
    <h2><%= pluralize(@post.errors.count, "error") %> prohibited this post from being saved:</h2>

    <ul>
    <% @post.errors.full_messages.each do |msg| %>
      <li><%= msg %></li>
    <% end %>
    </ul>
  </div>
<% end %>
```

Furthermore, if you use the Rails form helpers to generate your forms, when
a validation error occurs on a field, it will generate an extra `<div>` around
the entry.

```
<div class="field_with_errors">
 <input id="post_title" name="post[title]" size="30" type="text" value="">
</div>
```

You can then style this div however you'd like. The default scaffold that
Rails generates, for example, adds this CSS rule:

```
.field_with_errors {
  padding: 2px;
  background-color: red;
  display: table;
}
```

This means that any field with an error ends up with a 2 pixel red border.