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

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]

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

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

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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.

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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)

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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!

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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

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Most of the answers above should suffice you but if you are doing a lot more of such predicate and complex combinations check out Squeel. You will be able to doing something like:

Topic.where{{forum_id.not_in => @forums.map(&:id)}}
Topic.where{forum_id.not_in @forums.map(&:id)} 
Topic.where{forum_id << @forums.map(&:id)}

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You may want to have a look at the meta_where plugin by Ernie Miller. Your SQL statement:

SELECT * FROM topics WHERE forum_id NOT IN (<@forum ids>)

...could be expressed like this:

Topic.where(:forum_id.nin => @forum_ids)

Ryan Bates of Railscasts created a nice screencast explaining MetaWhere.

Not sure if this is what you're looking for but to my eyes it certainly looks better than an embedded SQL query.

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The original post specifically mentions using numeric IDs, but I came here looking for the syntax for doing a NOT IN with an array of strings.

ActiveRecord will handle that nicely for you too:

Thing.where(['state NOT IN (?)', %w{state1 state2}])

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Can these forum ids be worked out in a pragmatic way? e.g. can you find these forums somehow - if that is the case you should do something like

Topic.all(:joins => "left join forums on (forums.id = topics.forum_id and some_condition)", :conditions => "forums.id is null")

Which would be more efficient than doing an SQL not in

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This way optimizes for readability, but it's not as efficient in terms of database queries:

# Retrieve all topics, then use array subtraction to
# find the ones not in our list
Topic.all - @forums.map(&:id)

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You can use sql in your conditions:

Topic.find(:all, :conditions => [ "forum_id NOT IN (?)", @forums.map(&:id)])

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Piggybacking off of jonnii:

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

using pluck rather than mapping over the elements

found via railsconf 2012 10 things you did not know rails could do

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When you query a blank array add "<< 0" to the array in the where block so it doesn't return "NULL" and break the query.

Topic.where('id not in (?)',actions << 0)

If actions could be an empty or blank array.

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Here is a more complex "not in" query, using a subquery in rails 4 using squeel. Of course very slow compared to the equivalent sql, but hey, it works.

    scope :translations_not_in_english, ->(calmapp_version_id, language_iso_code){
      join_to_cavs_tls_arr(calmapp_version_id).
      joins_to_tl_arr.
      where{ tl1.iso_code == 'en' }.
      where{ cavtl1.calmapp_version_id == my{calmapp_version_id}}.
      where{ dot_key_code << (Translation.
        join_to_cavs_tls_arr(calmapp_version_id).
        joins_to_tl_arr.    
        where{ tl1.iso_code == my{language_iso_code} }.
        select{ "dot_key_code" }.all)}
    }

The first 2 methods in the scope are other scopes which declare the aliases cavtl1 and tl1. << is the not in operator in squeel.

Hope this helps someone.

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If someone want to use two or more conditions, you can do that:

your_array = [1,2,3,4]
your_string = "SOMETHING"

YourModel.where('variable1 NOT IN (?) AND variable2=(?)',Array.wrap(your_array),your_string)

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