How to express a NOT IN query with ActiveRecord/Rails?

Question

Rails 4+:

Article.where.not(title: ['Rails 3', 'Rails 5']) 

Rails 3:

Topic.where('id NOT IN (?)', Array.wrap(actions))

Where actions is an array with: [1,2,3,4,5]

FYI, In Rails 4, you can use not syntax:

Article.where.not(title: ['Rails 3', 'Rails 5'])

You can try something like:

Topic.find(:all, :conditions => ['forum_id not in (?)', @forums.map(&:id)])

You might need to do @forums.map(&:id).join(','). I can't remember if Rails will the argument into a CSV list if it is enumerable.

You could also do this:

# in topic.rb
named_scope :not_in_forums, lambda { |forums| { :conditions => ['forum_id not in (?)', forums.select(&:id).join(',')] }

# in your controller 
Topic.not_in_forums(@forums)

Using Arel:

topics=Topic.arel_table
Topic.where(topics[:forum_id].not_in(@forum_ids))

or, if preferred:

topics=Topic.arel_table
Topic.where(topics[:forum_id].in(@forum_ids).not)

and since rails 4 on:

topics=Topic.arel_table
Topic.where.not(topics[:forum_id].in(@forum_ids))

Please notice that eventually you do not want the forum_ids to be the ids list, but rather a subquery, if so then you should do something like this before getting the topics:

@forum_ids = Forum.where(/*whatever conditions are desirable*/).select(:id)

in this way you get everything in a single query: something like:

select * from topic 
where forum_id in (select id 
                   from forum 
                   where /*whatever conditions are desirable*/)

Also notice that eventually you do not want to do this, but rather a join - what might be more efficient.

To expand on @Trung Lê answer, in Rails 4 you can do the following:

Topic.where.not(forum_id:@forums.map(&:id))

And you could take it a step further. If you need to first filter for only published Topics and then filter out the ids you don't want, you could do this:

Topic.where(published:true).where.not(forum_id:@forums.map(&:id))

Rails 4 makes it so much easier!

The accepted solution fails if @forums is empty. To workaround this I had to do

Topic.find(:all, :conditions => ['forum_id not in (?)', (@forums.empty? ? '' : @forums.map(&:id))])

Or, if using Rails 3+:

Topic.where( 'forum_id not in (?)', (@forums.empty? ? '' : @forums.map(&:id)) ).all