如何为ActiveRecord模型设置model.changed？

Question

For any ActiveRecord model, I can access the changed attribute to get an array of fields that were changed.

For instance:

a = Article.find(1)
a.title = "New Title"
a.changed => returns ["title"]

Is there a way for me to set the "changed" attribute myself? Suppose I want to make it empty. Reason being, I'm actually reading the article attributes from a cache (Redis, but it doesn't matter which), instead of from the database.

Unfortunately, I can't do

def getArticleFromCache
    json = getJsonFromCache()
    a = Article.new
    a.attributes = json
    a.changed = [] #doesn't work, changed contains ALL the attributes in the json
    return a
end

What can I do?

In other words, I want someone who's calling getArticleFromCache to appear as if it's getting it from the database.

Answer 1

I seems that you basically want to clean the changed history.

For Rails 3, so you could use a.instance_variable_set(:@changed_attributes, {})

For Rails 4, you can just use a.clear_changes_information

Answer 2

You can do it via private methods from ActiveModel::Dirty

For rails 4 it is:

a.send(:clear_changes_information)

For rails 3 it is:

a.send(:reset_changes)

Obviously the methods being private gives a pretty big hint that they aren't designed to be called directly but it will work.

You may have more success, or a neater solution, by wrapping your Article in another class to decorate it. You could then override changes in that decorator class to indicate that there are no changes. You'll have to decide how much an article from the cache actually needs to behave like a model class. Do you want to save it, for example; you might end up with more problems than just trying to hide the changes.