Converting IDs into Models for a HABTM

Using Virtus to Coerce an Array of IDs into Objects/Models

You may at times need to coerce an array of model IDs into their corresponding models.

For example, imagine a Student has_and_belongs_to_many Schools. You may have a select box or similar form element on a StudentForm that includes a number of School id/name option pairings. Given this setup, when the form is submitted the response params would include a list of ids, e.g. "schools" => ["1", "3", "5", "7"].

In this situation it is easiest to make a custom Virtus::Attribute to use for your coercion, as in the following example:

class StudentForm < Reform::Form

  include Coercion

  class ArrayOfSchools < Virtus::Attribute
    # Takes in a list of school_ids (e.g., [1,3,5,7]) and returns a list of corresponding Schools
    def coerce(value)
      School.find(value.reject(&:empty?))
    end
  end

  property :schools, type: ArrayOfSchools
end

You can also easily add minimum/maximum validation using this approach.

# Ensure that at least 1 and no more than 4 schools were selected.
validates :schools, length: {minimum: 1, maximum: 4}