Feature request: Easier polymorphic joins

I recently had a problem (on Rails 4.1.2) trying to join on a polymorphic association where the polymorphic type was known ahead of time.

class EmailLog < ActiveRecord::Base

belongs_to :related_object

end

I had a query like this:

EmailLog.where(related_object_type: “FooBar”).joins(:related_object)

This fails because the related_object_type in the where doesn’t seem to carry through to the joins, and since related_object is a polymorphic association, it complains.

EmailLog.joins(:related_object).to_sql

ActiveRecord::EagerLoadPolymorphicError: Cannot eagerly load the polymorphic association :related_object

EmailLog.where(related_object_type: “FooBar”).joins(:related_object)

ActiveRecord::EagerLoadPolymorphicError: Cannot eagerly load the polymorphic association :related_object

I then discovered that there’s actually a way around this problem by specifying a relationship that piggy-backs on related_object:

belongs_to :foo_bar, -> { where(email_logs: { related_object_type: “FooBar” }) }, foreign_key: “related_object_id”

This is joinsable:

EmailLog.joins(:foo_bar).to_sql

#=> “SELECT email_logs.* FROM email_logs INNER JOIN foo_bars ON foo_bars.id = email_logs.related_object_id AND email_logs.related_object_type = ‘FooBar’”

It would be nice if the where(related_object_type: "FooBar") information could be carried forward to the joins call automatically, so that the original attempted query could work. I had originally thought that maybe a syntax like this would be useful:

EmailLog.joins(related_object: :foo_bar)

But then I realised that this obviously conflicts with the existing syntax for nested joins. I’m sure there’s something expressive that would be equivalent to defining the belongs_to :foo_bar and using EmailLog.joins(:foo_bar), though.

I’m not sure how possible this actually is. I also have no clue how to even start, otherwise I’d PR it!