Custom Rails Validators

How can I create my own validator?

For a few days I worked on custom validators in Rails. First what are validators? When you want to check some data which comes to your application, you use validators. For example:

  • if email has specific format,
  • if number is odd,
  • or if you simply want to check if name is required

for all of this we use validators.

Rails has many different validators already in side. Check the documentation. But sometimes you want to do more. In my case I need validator for black list of words for string field. I know I can use build-in validator. But I need much more customization:

  1. The list of excluded words was very long.
  2. I didn’t want to have this words in my Ruby file.
  3. I always want to add new wards without changing my application.

So I decide to do my own validator.

I look to documentation and I find this. I choosed ActiveModel::EachValidator where I have access to whole record, specific attribute to check and the value of this attribute. This was all I needed. To do my custom validator I must only write one method: validate_each. It look like this:

# app/validators/blacklist_validator.rb

# Validate list of words that can not be use in specifig field
class BlacklistValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    record.errors.add(attribute, :on_blacklist) if blacklist.include? value


  def blacklist
    File.readlines(Rails.root.join('config', 'blacklist.txt')).map(&:strip)

How this works?

I add error :on_blacklist to record when value is located on black list. I put my black list – blacklist.txt – in Rails config directory. To use this validator we put something like this in our model:

validates :name, blacklist: true

Remember convention:

When your valdator class is named BlacklistValidator then in your model you use blacklist: true parameter.

This was perfect for me, but what about put some custom parameters to validator? Something like this:

validates :age, numericality: { greater_than: 18 }

This is not a problem. In custom validators you have available something like options, where you have access to all custom parameters. When you call options for our example you see:

 => { greater_than: 18 }

I use this for array validator, where I check size of array:

# app/validators/array_lenght_validator.rb

# Validate if array is not too short
class ArrayLenghtValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    return unless options.key?(:minimum)
    array_size = (value.try(:size) || 0)
    minimum = options[:minimum]
    return if array_size >= minimum
    record.errors.add(attribute, :too_short_array, count: minimum)

What happened here?

I check if :minimum parameter is set. I count number of elements in array. If array_size is lower then minimum size, I add error :too_short_array to record.

I can call this validator like this:

validates :array, array_lenght: { minimum: 1 }

It is one more think to say. How put translation to our validators in locales?

We use Rails Convention. This is one but not only way to do this (all convention is explain in

              on_blacklist: is on words blacklist
                one: is too short (minimum is 1 item)
                other: is too short (minimum is %{count} items)

In our_model_name: key we put model name. For example when we have model User, we have key user:. Next we put names of our validated fields:

  • name: field (form black list validation),
  • array: field (from array length validation).

Then we put names of our errors: on_blacklist: and too_short_array:. For on_blacklist: this is it. But too_short_array: error had count parameter. Rails Internalization can recognized if it singular or plural version of count. And serve one: or other: translation. Last thing: In other: translation we put our count parameter thought %{}. Then Rails know where put count value.

This is it. I hope this was useful for you. Please live your comments, questions or any suggestions below. To next time! Bye!