Sorting has_many through on a join table property

Question

Let's say I have the following in a rails4 app:

class Foo < AR::Base
  has_many :foo_bar
  has_many :bar, through: :foo_bar
end

And the foo_bar table has a "sort" column with a number. To make matters even more entertaining, the "bar" table ALSO has a "sort" column with a number. Same name as the join table sort column. AND, this is important...

class Bar < AR::Base
  default_scope -> { order(:sort) }
end

I need to have a default scope on the association, such that:

foo = Foo.last
foo.bars # => List of bars, ordered by the number on the foo_bar table

How can I do this? It means unscoping the bar, but just the sort

Answer 1

You can specify the sort order for collection using order option

class Foo < AR::Base
  has_many :foo_bar
  has_many :bar, -> { order('foo_bar.sort') }, through: :foo_bar
end

Answer 2

You could do:

foo.bars.sort(foo_bar.position)

sort will sort the list by a given attribute, so give it the attribute on foo_bar that you wish to sort by.

Answer 3

I got it to work by doing this:

class Foo < AR::Base
  has_many :foo_bar
  has_many :bar, through: :foo_bar

  def sorted_bars
    foo_bar.map(&:bar)
  end
end

Because of the default scopes, this was too complicated for me. I'll still accept an answer that offers a more elegant solution :)