Question

I have an Entry model which has many Tags. Tags are added to an entry by typing them into a textbox on my form, via a tag_names virtual attribute. Before validation on the Entry model, the tag_names string is converted into actual Tag objects using find_or_create_by_name. The Tag model also has validation to make sure the tag name matches a regex, which is run via the association.

My Entry model looks like this:

class Entry < ActiveRecord::Base
  has_many :entry_tags
  has_many :tags, :through => :entry_tags

  before_validation :update_tags

  attr_writer :tag_names

private
  def update_tags
    if @tag_names
      self.tags = @tag_names.split(",").uniq.map do |name|
        Tag.find_or_create_by_name(name.strip)
      end
    end
  end
end

When I create a new Entry object and assign it tags, everything works correctly — the tags are not saved if there is a validation error on one of the Tags, and an error message is passed back. However, if I try to update an existing Entry object with an invalid tag, instead of passing back a message, my self.tags= call (in update_tags above) is throwing an exception with the validation error message. Even if I overwrite find_or_create_by_name to actually just return a new object instead of calling create, I get the same outcome.

It seems to me (and the docs seem to corroborate) that the tags= call is actually saving my Tag objects before the main record gets saved when the Entry object already exists. Is there anything I can do to make this save not happen, or to stop it from raising an exception and just causing my save to return false?

Answer 1

I would try something like this:

class Entry < ActiveRecord::Base
  has_many :entry_tags
  has_many :tags, :through => :entry_tags

  before_validation :update_tags

  attr_writer :tag_names
  validates_associated :tags

private
  def update_tags
    return unless @tag_names
    current_tag_names = tags.map(&:name)
    user_tag_names = @tag_names.split(",").uniq
    #add in new tags
    user_tag_names.each do |name|
      next if current_tag_names.include?(name)
      tags.build :name => name
    end
    #remove dropped tags
    ( current_tag_names - user_tag_names ).each do |name|
      removed_tag = tags.find_by_name(name)
      tags.delete(removed_tag)
    end
  end
end

This way you’re only initializing the related models in your update_tags action and so won’t throw validation errors. I also added in the validates_associated :tags so that errors on these related models can be reported back via the standard input form using error_messages_for :entry.

Update included code for removing dropped tags.