Rails ActiveRecord: validate single attribute

Question

Is there any way I can validate a single attribute in Rails?

Something like:

ac_object.valid?(attribute_name) 

Answer 1

Sometimes there are validations that are quite expensive (e.g. validations that need to perform database queries). In that case you need to avoid using valid? because it simply does a lot more than you need.

There is an alternative solution. You can use the validators_on method of ActiveModel::Validations.

validators_on(*attributes) public

List all validators that are being used to validate a specific attribute.

according to which you can manually validate for the attributes you want

e.g. we only want to validate the title of Post:

class Post < ActiveRecord::Base    validates :body, caps_off: true    validates :body, no_swearing: true   validates :body, spell_check_ok: true    validates presence_of: :title   validates length_of: :title, minimum: 30 end 

Where no_swearing and spell_check_ok are complex methods that are extremely expensive.

We can do the following:

def validate_title(a_title)   Post.validators_on(:title).each do |validator|     validator.validate_each(self, :title, a_title)   end end 

which will validate only the title attribute without invoking any other validations.

p = Post.new p.validate_title("") p.errors.messages #=> {:title => ["title can not be empty"] 

note

I am not completely confident that we are supposed to use validators_on safely so I would consider handling an exception in a sane way in validates_title.

Answer 2

You can implement your own method in your model. Something like this

def valid_attribute?(attribute_name)   self.valid?   self.errors[attribute_name].blank? end 

Or add it to ActiveRecord::Base