Add ActiveModel::Errors#details

To be able to return type of validator, one can now call `details`
on Errors instance:

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

```ruby
user = User.new; user.valid?; user.errors.details
=> {name: [{error: :blank}]}
```

1 files changed, 56 insertions, 2 deletions

diff --git a/guides/source/active_record_validations.md b/guides/source/active_record_validations.md
index c9af70934a..84925072f2 100644
--- a/guides/source/active_record_validations.md
+++ b/guides/source/active_record_validations.md
@@ -227,8 +227,26 @@ end
 ```
 
 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.
+Errors](#working-with-validation-errors) section.
+
+### `errors.details`
+
+To check what validator type was used on invalid attribute, you can use
+`errors.details[:attribute]`. It returns array of hashes where under `:error`
+ key you will find symbol of used validator.
+
+```ruby
+class Person < ActiveRecord::Base
+  validates :name, presence: true
+end
+
+>> person = Person.new
+>> person.valid?
+>> person.errors.details[:name] #=> [{error: :blank}]
+```
+
+Using `details` with custom validators are covered in the [Working with
+Validation Errors](#working-with-validation-errors) section.
 
 Validation Helpers
 ------------------
@@ -1074,6 +1092,42 @@ Another way to do this is using `[]=` setter
    # => ["Name cannot contain the characters !@#%*()_-+="]
 ```
 
+### `errors.details`
+
+You can add validator type to details hash when using `errors.add` method.
+
+```ruby
+  class Person < ActiveRecord::Base
+    def a_method_used_for_validation_purposes
+      errors.add(:name, :invalid_characters)
+    end
+  end
+
+  person = Person.create(name: "!@#")
+
+  person.errors.details[:name]
+   # => [{error: :invalid_characters}]
+```
+
+To improve error details to contain not allowed characters set, you can
+pass additional options to `errors.add` method.
+
+```ruby
+  class Person < ActiveRecord::Base
+    def a_method_used_for_validation_purposes
+      errors.add(:name, :invalid_characters, not_allowed: "!@#%*()_-+=")
+    end
+  end
+
+  person = Person.create(name: "!@#")
+
+  person.errors.details[:name]
+  # => [{error: :invalid_characters, not_allowed: "!@#%*()_-+="}]
+```
+
+All built in Rails validators populate details hash with corresponding
+validator types.
+
 ### `errors[:base]`
 
 You can add error messages that are related to the object's state as a whole, instead of being related to a specific attribute. You can use this method when you want to say that the object is invalid, no matter the values of its attributes. Since `errors[:base]` is an array, you can simply add a string to it and it will be used as an error message.