Is there any way to use AREL for custom associations?

问题

model Post
  # ActiveRecord associations have tons of options that let
  # you do just about anything like:
  has_many :comments
  has_many :spam_comments, :conditions => ['spammy = ?', true]

  # In Rails 3, named scopes are ultra-elegant, and let you do things like:
  scope :with_comments, joins(:comments)
end

Is there any way to use AREL, or an otherwise leaner syntax, to define custom associations as elegantly as named scopes?

update

I've decided it's not a good idea to put that sort of detail into an association anyway, because associations should always/mostly define the basic relationships between models.

回答1:

One of the solutions is to put spammy scope on Comments:

model Post
  has_many :comments

  scope :with_comments, joins(:comments)
end

model Comment
  scope :spammy, where(:spammy => true)
end

This looks a bit cleaner with respect to model responsibilities. Performance-wise it's exactly the same:

p.comments.spammy.to_sql
# → SELECT "comments".* FROM "comments"
#   WHERE ("comments".post_id = 2) AND ("comments"."spammy" = "t")

The added benefit: you can get spammy comments from any other associations.

回答2:

Well, there may be a better way, but I know that you can use actual Arel conditions (as opposed to ActiveRecord::Relations) in associations by using Arel's to_sql feature.

has_many :spam_comments, :class_name => 'Comment', :conditions => Comment.arel_table[:spammy].eq(true).to_sql

You'll notice that actual Arel code isn't as lean as ActiveRecord Relations.

I did find a comment in the Rails master branch that referred to passing an Arel predicate as a condition but that code doesn't seem to be in the 3.0 branch. At least not that I could find.

来源：https://stackoverflow.com/questions/5783853/is-there-any-way-to-use-arel-for-custom-associations