使用ActiveRecord自行加入桌面

Question

I have an ActiveRecord called Name which contains names in various Languages .

class Name < ActiveRecord::Base
  belongs_to :language

class Language < ActiveRecord::Base
  has_many :names

Finding names in one language is easy enough:

Language.find(1).names.find(whatever)

But I need to find matching pairs where both language 1 and language 2 have the same name. In SQL, this calls for a simple self-join:

SELECT n1.id,n2.id FROM names AS n1, names AS n2
  WHERE n1.language_id=1 AND n2.language_id=2
    AND n1.normalized=n2.normalized AND n1.id != n2.id;

How can I do a query like this with ActiveRecord? Note that I need to find pairs of names (= both sides of the match), not just a list of names in language 1 that happens to match with something.

For bonus points, replace n1.normalized=n2.normalized with n1.normalized LIKE n2.normalized , since the field may contain SQL wildcards.

I'm also open to ideas about modeling the data differently, but I'd prefer to avoid having separate tables for each language if I can.

Answer 1

Try this:

ids = [1,2]
Name.all(:select    => "names.id, n2.id AS id2",
         :joins     => "JOIN names AS n2 
                              ON n2.normalized = names.normalized AND 
                                 n2.language_id != names.language_id AND
                                 n2.language_id IN (%s)" % ids.join(','),
         :conditions => ["names.language_id IN (?)", ids]
).each do |name|
  p "id1 : #{name.id}"
  p "id2 : #{name.id2}"
end

PS: Make sure you sanitize the parameters passed to the join condition.

Answer 2

It sounds like you might want to use a many-to-many relationship between Language and Name instead of has_many/belongs_to.

>> Language.create(:name => 'English')
 => #<Language id: 3, name: "English", created_at: "2010-09-04 19:15:11", updated_at: "2010-09-04 19:15:11"> 
>> Language.create(:name => 'French')
 => #<Language id: 4, name: "French", created_at: "2010-09-04 19:15:13", updated_at: "2010-09-04 19:15:13"> 
>> Language.first.names << Name.find_or_create_by_name('Dave')
 => [#<Name id: 3, name: "Dave", language_id: 3, created_at: "2010-09-04 19:16:50", updated_at: "2010-09-04 19:16:50">] 
>> Language.last.names << Name.find_or_create_by_name('Dave')
 => [#<Name id: 3, name: "Dave", language_id: 4, created_at: "2010-09-04 19:16:50", updated_at: "2010-09-04 19:16:50">]
>> Language.first.names.first.languages.map(&:name)
 => ["English", "French"] 

This extra level of normalization should make what you are trying to do easier.