Ruby on Rails is beautiful! It does seem confusing at the beginning but it offers quick solutions to issues which may get quite complicated in some other development frameworks.
I have a table relation where multiple columns of the same table refer to the same foreign key. In this case has_many and belongs_to does not directly cut it. There is, however, a very easy solution.
Here is the situation, I have a table which lists languages
languages
| id | name |
|---|---|
| 1 | English |
| 2 | Polish |
| . | . |
| . | . |
Then I have the table which lists the language pairs for a user:
language_pairs
| user_id | source_language_id | target_language_id |
|---|---|---|
| 1 | 1 | 2 |
| 1 | 2 | 3 |
| . | . | . |
| . | . | . |
As you guessed, both the source_language and target_language columns map to the same key, the id column of the languages table. This is how you map this relation in rails:
language.rb
has_many :occurances_as_target_language, :class_name => “LanguagePair”, :foreign_key => “target_language_id”
end
language_pair.rb
belongs_to :target_language, :class_name => “Language”, :foreign_key => “target_language_id
end
By doing this I am able to map both columns that I have in my language pairs table to the languages table’s id column. After this, things are super easy and instances below work as expected:
@language_pair.source_language.name #returns the name of the language that corresponds to source_language_id
@language_pair.target_language.name #returns the name of the language that corresponds to target_language_id
Multiple foreign keys and Rails? Easy! Happy coding!
hello
thanks for making this post
I have a question.
How did you define two model, language and language pair in db?(db-migration files)
Jongwon, this was just example that I used to explain the situation and the required models. I do not have an actual database migration for this one. What is the problem that you are having?
it’d be great if u explained what the migration would look like as well 😉 i got as far as mapping at the class level, but not having any luck with the actual migration 🙁
oh nevermind, i got it ;D thx man
Hi! Thanks for the post, it helped me a lot. 🙂
Although I would like to ask you if you know of a “better” way to do something like this? I’m thinking of something that would use joins (your approach uses separate SELECT queries). If you had a list of 20 language pairs, your approach would create 20 x 2 SELECT queries.
With a few exceptions of course – some Languages will be loaded from cache if they were fetched previously on the same page load.
Thanks!
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