Question

I am using Ruby on Rails 3.0.10 and I would like to “subtract” two ActiveRecord::Relation “entities” and have back again an ActiveRecord::Relation. That is, I have two ActiveRecord::Relation objects (@articles and @articles_checked) and the following code:

@unchecked_articles = @articles - @articles_checked

# $  @unchecked_articles.class
# => Array

@unchecked_articles.method_call
# raise a NoMethodError error (read above for more information).

The above code, anyway, returns a Ruby Array so I can not “play” (use where, order, … statements) anymore with that as I do normally with an ActiveRecord::Relation. In the console it generates the following error:

NoMethodError (undefined method 'method_call' for #<Array:0x000001063dd658>)

How can I retrieve an ActiveRecord::Relation object after I made the above changes? Or better, there is a way to accomplish what I aim to retrieve with the @unchecked_articles?

Answer 1

I don’t know how your model looks, but i’d suggest defining a scope for articles.

Article < ActiveRecord::Base
  scope :unchecked, where(:checked => false)

  #or this, apologies for somewhat unefficient, but you already seem to have several queries
  scope :unchecked2, lambda { |checked| where(["id not in (?)", checked.map(&:id)]) }

end

Article.unchecked
Article.unchecked2(@unchecked_articles)