ActiveModel supplies an errors object that behaves similar to a Hash. It can be used to add errors to a record, as well as to query the record for registered errors. This object is returned when calling <object>.errors:

errors = @user.errors # => #<ActiveModel::Errors ...>

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Here are some helpful messages of its API:

[<attribute name>]
: Returns an array of error messages on that attribute. Example: errors[:name] => ['is missing']

add_on_blank(<attribute list>) (similarly add_on_empty)
: Registers an error for each given attribute if it is blank?. Example: errors.add_on_blank(:name, :email, :address)

generate_message(<attribute>, <error type, e.g. :invalid>)
: Generates an error message for the given attribute. Will retrieve the error message from the locale files Show archive.org snapshot .

get(<attribute>)
: (Almost) an alias of []. Calls messages[<attribute>] and returns an array of error messages for that attribute.

include?(<attribute>)
: Returns whether there is an error message for that attribute.

set(<attribute>, <value>)
: Sets the error messages for attribute to value. It should be an array of error messages, but can technically be any object.

to_hash(<want full error messages prefixed with attribute name?>)
: Returns a hash of attribute: error_messages.