On Tuesday, May 12, 2015 at 4:42:46 PM UTC-7, Janko Marohnić wrote:
>
> A question related to this situation, which I'm not sure it's a bug.
>
> user = User.eager(:quizzes => :questions).all.first
> user.associations #=> {:quizzes => [...]}
>
> user = User.eager(:quizzes => :questions).first
> user.associations #=> {}
>
> So, when I fetch the first user directly, the eager loading is cancelled,
> meaning that fetching questions for each quiz will result in a N+1 query.
> However, doesn't Sequel have all the necessary information to do the eager
> loading?
>
First, if you are loading a single record, there is no reason to use eager
loading. Eager loading is only an advantage if you are loading multiple
records. 1+1 = N+1 if N = 1.
In terms of having the necessary information to eager load, not really.
Dataset#first calls Dataset#each, which iterates over the recordset, and
therefore doesn't have all records available before iterating, which is
necessary to eager load correctly. If you want, you can use the eager_each
plugin, which makes #each call #all internally if the dataset is supposed
to eager load.
So no, this isn't a bug, it is expected behavior.
If you want to always eager load questions when loading quizzes for a given
user:
User.first.quizzes{|ds| ds.eager(:questions)}
# or
User.one_to_many :quizzes, :eager=>:questions
User.first.quizzes # eager loads questions as well
Thanks,
Jeremy
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