Self-join on a table with ActiveRecord

问题

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.

回答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.

回答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.

来源：https://stackoverflow.com/questions/3642005/self-join-on-a-table-with-activerecord

标签

ruby-on-rails

activerecord

mysql

self-join