loading ActiveRecords via a complex query

rubyonrails-talk
1

Ok, I have a many_to_many relationship between two tables (A, B) implemented by a join table (J). J has two columns: a_id and b_id.

Currently, I use a query like this: <code> recs = A.find :all, :select => 'A.*',                            :joins => ', B, J',                            :conditions => 'A.id = J.a_id AND J.b_id = B.id AND B.somefield = somevalue ' </code>

That works well, but is there a more Rails like way to do it?

Also, is there a more efficient way to do it? Notice in my RoR code, I'm joining 3 tables together in a single SQL statement. Would it be faster to select ids in an SQL call joining only 2 tables, then calling A.find :all, ids?

In code: <code> ids_array = "SELECT J.a_id FROM J, B WHERE J.b_id = B.id AND B.somefield = somevalue" recs = A.find :all, ids_array </code>

How do I efficiently execute that query and get an array of ids in return?

Thanks for the help.

2

You have two options.

has_and_belongs_to_many :foo

Each of the two models that's gonna be joined has a has_and_belongs_to_many :other_model in it. Say your models are called Book and Author - the table would then be named authors_books (plural and in alphabetic order). The authors_books table would have two columns: book_id and author_id. No primary key.

has_many :foo, :through => :bar

A bit to intricate to explain. Look at this pastie: http://pastie.caboo.se/34094

3

I know how join tables work pertaining to Rails models. I'm asking about how to load records using complex queries that involve join tables.

See my pastie: http://pastie.caboo.se/34129

Thanks.