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 `joins`able:
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!
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