How can I use a named scope in my model against an array of items?

Question

I know that I can do a query for recent books based on an array as in

scope :recent_books, lambda {|since_dt| {:conditions=>{:created_at >= since_dt}}}

Answer 1

If you pass an array as the value, ActiveRecord is smart enough to compare for inclusion in the array. For example,

Book.where(:author_id => [1, 7, 42])

produces a SQL query with a WHERE clause similar to:

WHERE "author_id" IN (1, 7, 42)

You can take advantage of this in a scope the same way you would set normal conditions:

class Book < ....
  # Rails 3
  scope :by_author, lambda { |author_id| where(:author_id => author_id) }

  # Rails 2
  named_scope :by_author, lambda { |author_id
    { :conditions => {:author_id => author_id} }
  }
end

Then you can pass a single ID or an array of IDs to by_author and it will just work:

Book.by_author([1,7,42])

Answer 2

In Rails 4, I can check for the inclusion of a string in an array attribute by using a scope like this:

scope :news, -> { where(:categories => '{news}') }

Or with an argument:

scope :by_category, ->(category) { where(:categories => "{#{category}}") }