How to use Rails #update_attribute with array field?

I don't think update_attribute is going to be useful as it will replace the array with the new value rather than append to it (but see better explanation below in --Update-- section).

I'm not sure what best practices are here, but this should work to just add something if it isn't already there:

question.tags << :ruby unless question.tags.include?(:ruby)
question.save

I would write a custom method on the Question model to manage adding tags and checking for uniqueness:

def add_tag(tag)
  if tags.include?(tag)
    # do whatever you want to do when tag isn't unique
    p "#{tag} is already in tags!"
  else
    tags << tag
    save
  end
end

Then call it with question.add_tag(:ruby).

-------------------- Update -----------------------

If this doesn't work, there could be an issue with ActiveRecord not recognizing the field has changed (although it seems to work OK in Rails 4.2).

These links explain the issue:

New data not persisting to Rails array column on Postgres

http://paweljaniak.co.za/2013/07/28/rails-4-and-postgres-arrays/

As they explain, you can use update_attribute here, but you need to replace the entire array rather than push a value onto it, like this:

question.update_attribute(:tags, question.tags << tag)

You should also be able to force ActiveRecord to consider the attribute updated by including will_change! like this:

def add_tag(tag)
  if tags.include?(tag)
    # do whatever you want to do when tag isn't unique
    p "#{tag} is already in tags!"
  else
    tags_will_change!
    tags << tag
    save
  end
end