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Fix #12537 performance regression when preloading has_many_through association #19423

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merged 1 commit into from Dec 19, 2015
Merged

Fix #12537 performance regression when preloading has_many_through association #19423

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9 changes: 7 additions & 2 deletions activerecord/lib/active_record/associations/preloader/through_association.rb
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Expand Up @@ -18,7 +18,8 @@ def associated_records_by_owner(preloader)
through_records = owners.map do |owner|
association = owner.association through_reflection.name

[owner, Array(association.reader)]
center = target_records_from_association(association)
[owner, Array(center)]
end

reset_association owners, through_reflection.name
Expand Down Expand Up @@ -49,7 +50,7 @@ def associated_records_by_owner(preloader)
rhs_records = middles.flat_map { |r|
association = r.association source_reflection.name

association.reader
target_records_from_association(association)
}.compact

rhs_records.sort_by { |rhs| record_offset[rhs] }
Expand Down Expand Up @@ -89,6 +90,10 @@ def through_scope

scope
end

def target_records_from_association(association)
association.loaded? ? association.target : association.reader
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@yuroyoro why do you check against loaded? here? It seems that association is always loaded in this case and all tests remain green when I delete it. Can you add a test that fails without it?

end
end
end
end
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6 changes: 6 additions & 0 deletions activerecord/test/cases/associations/eager_test.rb
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Expand Up @@ -1349,4 +1349,10 @@ def test_deep_including_through_habtm
post = Post.eager_load(:tags).where('tags.name = ?', 'General').first
assert_equal posts(:welcome), post
end

# CollectionProxy#reader is expensive, so the preloader avoids calling it.
test "preloading has_many_through association avoids calling association.reader" do
ActiveRecord::Associations::HasManyAssociation.any_instance.expects(:reader).never
Author.preload(:readonly_comments).first!
end
end