Keeping custom Rails model validations DRY

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There are a few blog posts on writing model validators – this is another one which follows the DRY (don’t repeat yourself) philosophy.

Suppose you want to check that an attribute value is present in a table of tags in our database. Now, this isn’t a particularly good use case as you could use validates_inclusion_of, but lets assume that we want to DRY that up a bit.

Let’s write the validation to make the following work:

validates_inclusion_in_tags :primary_tag, :secondary_tag

The validation method would look a little bit like this:

def validates_inclusion_in_tags(*attr_names)
  validates_each attr_names do |record, attr_name, value|
    acceptable_values = Tag.find.all.map{ |obj| obj.name }
    record.errors.add(attr_name, "does not match any available values") unless acceptable_values.include?(value)
  end
end

What’s going on here? Firstly we attach our custom validation to the model using validates_each. This runs the block for each of the attribute names (symbols) in the attr_names array (note that we have used the splat operator in our argument list to flatten the arguments into an array so we can pass a list of attribute to validate). The validation block is passed three arguments: the record being validated, the attribute name (as a symbol) and the current value of the attribute that is being validated. We then execute a find on our Tag model to get an array of AR records which we then map to extract the name of each object into an array. Finally we associate an error to the attribute that we are validating if its value is not in the list of acceptable values we have created.

To make the validation method a little more configurable and a little more generic we can allow the validation method to be called with a hash of options. Maybe we want to define a different error message, supply a different model to validate against, or supply a different attribute to be selected from our model. This can be easily achieved like so:

def validates_inclusion_in_model(*attr_names)
  # Set up any default configuration options and merge on any passed to the validation
  configuration = {
    :message => "does not match any available values",
    :model => Tag,
    :model_attribute => :name,
  }.merge(attr_names.extract_options!)

  validates_each attr_names do |record, attr_name, value|
    acceptable_values = configuration[:model].find.all.map{ |obj| obj.send configuration[:model_attribute] }
    record.errors.add(attr_name, configuration[:message]) unless acceptable_values.include?(value)
  end
end

You’ll note that the default configuration options are merged with the configuration options that were passed into the validation method. This (very common) technique makes use of the extract_options! method which returns either the last item of an array if it is a Hash or an empty Hash.

OK, that’s all good – what’s the best way to get this method available to our models. The best ways are as follows: Either create a validators.rb file in config/initializers and open up the ActiveRecord::Base class like so:

ActiveRecord::Base.class_eval do
  # define your validation methods here
end

Or, the way I prefer is to to create a validations.rb file in your lib directory and use the following code:

module Validations
  # Extend the caller with the ClassMethods module
  def self.included(base) # :nodoc:
    base.extend ClassMethods
  end

  module ClassMethods
    # define your validation methods here
  end
end

Now you can pick and chose which validations are available to your models rather than extend every model that inherits from AR Base by using:

class Person < ActiveRecord::Base
  include Validations
end

Of course, if all of your AR models inherit from a shared base class which in turn inherits from AR Base, then you could also include the validations model at your shared base model level to give all your models access to your new validations.

Note on performance

The validation method here would load all the Tag records into AR objects in memory just to throw them away again. Also, this problem is exacerbated if you were to compare two attributes against the list as the objects are loaded for each call into validates_each.