Rails Ruby on Rails

Eager loading a has_one with :order is not consistent.

Post.has_many :comments

Post.has_one :latest_comment, :class_name => 'Comment', :order => 'created_at desc'

(let's assume that comment ids increase in step with created_at)

Post.find(:all).latest_comment.id #=> 5

Post.find(:all, :include => :latest_comment).latest_comment.id #=> 4

Post.find(:all, :include => :latest_comment, :conditions => 'comments.id is not null').latest_comment.id #=> all bets are off - fallback to old eager loading ignores latest_comment :order

The inconsistency for the non-fallback eager loading stems from the fact that the set_xxx_target method is called for each relevant result, ultimately favoring the last record in the set.

The non-eager load will :limit to 1 record, favoring the first record in the set.

Sadly, the strategy of making new-style eager loading the default (rather than an explicit option to find()) and having an automatic fallback to the old-style eager loading leaves no clear-cut winner here.

That said, when comparing fallback-eager's "all bets are off" to new-eager's "the exact opposite of what you asked for", I'd say new-eager makes the least sense.

The preloading call to set_xxx_target for has_one associations should use the first relevant record and discard the rest.