Associations

I ran into an issue I had never faced recently when validating changes made to associated models.

Context:

Let’s suppose we have the following models (non-related info removed):

class User < ApplicationRecord
  has_many :rules
  has_many :activities
end

class Rule < ApplicationRecord
  belongs_to :user
  def check!(activity)
    #PSEUDO CODE
    unless allowed_actions.include?(activity.action) && allowed_tags.include?(activity.tags)
      activity.add_error("Not allowed to do this activity")
    end
  end
end

class Activity < ApplicationRecord
   belongs_to :user
   has_and_belongs_to_many :tags
   validates_with ActivityValidator
end

class Tag < ApplicationRecord
   has_and_belongs_to_many :activities
   # Tags contain information regarding the activity and rules can 'invalidate' an activity based on certain tags
   # E.g Rule does not allow User X to do any activities tagged with 'dangerous'
end

class ActivityValidator < ActiveModel::Validator
  IGNORED_ATTRIBUTES = ["status"]
  def validate(activity)
    # Allow certain changes without validating rules again
    no_changes_present   = (activity.changed_attributes.keys - IGNORED_ATTRIBUTES).empty? 
    return if no_changes_present
    activity.user.rules.each do |rule|
      rule.check!(activity)
    end
  end
end

Issue

Maybe the more seasoned Rails devs have already noticed the problem.

The issue I faced was that when changing any of the attributes on the activity (excluding the ignored status) the Rule was correctly evaluated. However, if there was a Rule which was based on checking Tags when making any changes to just the associated tags the validator passed even with ‘invalid’ Tags.

What was going on? Why aren’t my associations being validated?

First port of call was to slap in a binding.pry to see what no_changes_present and to my suprise it was returning true even when I was either deleting or adding Tags.

It was at this point I found out that activity.changed_attributes (and by extension #changes) ONLY covers changes to that model’s direct attributes and does not include changes to any associated models. Docs for changed_attributes

Yea I know a real ‘duh’ moment (the clue is in the name of the method) but due to less than perfect tests (which I will write more on in another post) this went unnoticed for too long.

Solution

The solution I chose is pretty simple, add the following to the no_changes_present check.

&& !activity.tags.any? { |tag| tag.marked_for_destruction? || tag.new_record? }

Now if a Tag is added (new_record?) or removed (marked_for_destruction?) no_changes_present will now be false and the validation will be carried out as normal.

One gotcha to remember is that we need to do a similar thing for the check! method on the Rule. However this time we only want to validate against Tags that are already associated or newly associated so:

def check!(activity)
  #PSEUDO CODE
  active_tags = activity.tags.reject { | tag | tag.marked_for_destruction? }
  unless allowed_actions.include?(activity.action) && allowed_tags.include?(active_tags)
    activity.add_error("Not allowed to do this activity")
  end
end

This way we can remove the due to be deleted tags to give us a correct validation before saving the edited Activity.

Now I am aware there are probably a million ways I could’ve achieved the same result so I would love to hear from anyone out there who has any suggestions on how else this could be implemented.

Til next time..

Dude where’s my association?

Context:

Issue

Solution