Why does ActiveRecord not use the primary_key and foreign_key columns when generating the SQL for a join in an association?

Question

I know I can use :finder_sql to manually define the SQL to use to fetch associated records, but I'm wondering if ActiveRecord uses the :primary_key and :foreign_key options on an association to generate the joining SQL. It doesn't appear to, but am I just missing something here?

---

Update: To be more explicit, my question is: Is there some way to keep the id columns as the primary key but have AR use a different column for the joins when I specify my relationships?

---

Here's example model definitions and an example console session...

Models

class Post < ActiveRecord::Base
  #Columns are: id:integer uuid:string name:string
  has_many :assignments, :foreign_key => 'post_uuid', :primary_key => 'uuid'
  has_many :categories, :through => :assignments
end

class Category < ActiveRecord::Base
  #Columns are id:integer uuid:string name:string
  has_many :assignments, :foreign_key => 'category_uuid', :primary_key => 'uuid'
end

class Assignment < ActiveRecord::Base
  #Columns are post_uuid:string category_uuid:string
  belongs_to :post, :foreign_key => 'uuid', :primary_key => 'post_uuid'
  belongs_to :category, :foreign_key => 'uuid', :primary_key => 'category_uuid'
end

Console Session

#Make a Post
>> p = Post.create(:uuid => '123', :name => 'The Post')
  Post Create (0.9ms)   INSERT INTO "posts" ("name", "created_at", "uuid", "updated_at") VALUES('The Post', '2010-02-04 00:05:13', '123', '2010-02-04 00:05:13')
=> #<Post id: 2, uuid: "123", name: "The Post", created_at: "2010-02-04 00:05:13", updated_at: "2010-02-04 00:05:13">

#Make a Category
>> c = Category.create(:uuid => '456', :name => 'The Category')
  Category Create (0.5ms)   INSERT INTO "categories" ("name", "created_at", "uuid", "updated_at") VALUES('The Category', '2010-02-04 00:05:30', '456', '2010-02-04 00:05:30')
=> #<Category id: 2, name: "The Category", uuid: "456", created_at: "2010-02-04 00:05:30", updated_at: "2010-02-04 00:05:30">

#Make an Assignment, associating the post and the category
>> a = Assignment.create(:post_uuid => p.uuid, :category_uuid => c.uuid)
  Assignment Create (0.4ms)   INSERT INTO "assignments" ("created_at", "updated_at", "post_uuid", "category_uuid") VALUES('2010-02-04 00:05:50', '2010-02-04 00:05:50', '123', '456')
=> #<Assignment id: 2, post_uuid: "123", category_uuid: "456", created_at: "2010-02-04 00:05:50", updated_at: "2010-02-04 00:05:50">

#Try to fetch the category from the post object, which generates the wrong SQL
>> p.categories
  Category Load (0.0ms)   SQLite3::SQLException: no such column: assignments.uuid: SELECT "categories".* FROM "categories" INNER JOIN "assignments" ON "categories".id = "assignments".uuid WHERE (("assignments".post_uuid = 2)) 
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: assignments.uuid: SELECT "categories".* FROM "categories"  INNER JOIN "assignments" ON "categories".id = "assignments".uuid    WHERE (("assignments".post_uuid = 2)) 
[...Stack trace edited out...]

#Also odd: Try to fetch the post from the Association, no SQL generated, we just get nil
>> a.post
=> nil

Answer 1

I was able to get your code to work by changing your model definitions.

class Post < ActiveRecord::Base
  set_primary_key :uuid  # tell the model about the non standard primary key
  has_many :assignments, :foreign_key => 'post_uuid'
  has_many :categories, :through => :assignments
end

class Category < ActiveRecord::Base
 set_primary_key :uuid # tell the model about the non standard primary key
 has_many :assignments, :foreign_key => 'category_uuid'
end

class Assignment < ActiveRecord::Base
  belongs_to :post, :foreign_key => 'post_uuid' # notice the value for the foreign_key
  belongs_to :category, :foreign_key => 'category_uuid'  # notice the value for the foreign_key
end

Apart from this, change the migration file to nominate the non standard primary key(uuid) in Category and Post models.

class CreatePosts < ActiveRecord::Migration
  def self.up
    create_table :posts, {:id => false} do |t|
      t.string :uuid, :primary => true, :limit => 20 
      t.string :name
      t.timestamps
    end
  end

  def self.down
    drop_table :posts
  end
end

class CreateCategories < ActiveRecord::Migration
  def self.up
    create_table :categories, {:id => false} do |t|
      t.string :uuid, :primary => true, :limit => 20
      t.string :name

      t.timestamps
    end
  end

  def self.down
    drop_table :categories
  end
end

You can't set the value for uuid via mass assignments while calling new/create method. You have to set the value for the primary_key explicitly.

 # this will not work. uuid will be set to 0
 p = Post.create(:uuid => '123', :name => 'The Post') 

 # this will work
 p = Post.new(:name => 'The Post')
 p.uuid = '123'
 p.save 

Answer 2

Did you ever find a solution to this problem? I want to do the same thing and AR still doesn't seem to honor the primary_key parameter.

Using the above approach, the id of each return object will be messed with, which isn't ideal when manipulating data.