Consistency between first() and last() with limit #27597

brchristian · 2017-01-06T22:34:28Z

As discussed in #23979, there was an inconsistency between the way that first() and last() would interact with limit. Specifically:

> Topic.limit(1).first(2).size
=> 2
> Topic.limit(1).last(2).size
=> 1

This PR is a refactor and rebase of #24124, with a simpler test suite and simpler implementation.

Discussion with Rails community members as well as DHH in #23598 (comment) showed that the behavior or first should be brought into line with last (rather than vice-versa).

This PR resolves the inconsistency between first and last when used in conjunction with limit.

rails-bot · 2017-01-06T22:34:41Z

r? @pixeltrix

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kamipo · 2018-01-07T10:42:14Z

Can you rebase?

brchristian · 2018-01-08T01:01:55Z

@kamipo Rebased to latest master 👍

kamipo · 2018-01-08T06:02:45Z

activerecord/lib/active_record/relation/finder_methods.rb

+
+            if limit_value.present?
+              limit = [limit_value - index, limit].min
+              limit = [limit, 0].max


If limit <= 0, we don't need a query to database.

diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb index e61cacf6a7..50d0f14b98 100644 --- a/activerecord/lib/active_record/relation/finder_methods.rb +++ b/activerecord/lib/active_record/relation/finder_methods.rb @@ -532,7 +532,11 @@ def find_nth_with_limit(index, limit) else relation = ordered_relation - if limit_value.nil? || index < limit_value + if limit_value + limit = [limit_value - index, limit].min + end + + if limit > 0 relation = relation.offset(offset_index + index) unless index.zero? relation.limit(limit).to_a else

Great point! I have updated the PR.

kamipo · 2018-01-08T06:40:15Z

activerecord/test/cases/finder_test.rb

@@ -693,6 +693,11 @@ def test_take_and_first_and_last_with_integer_should_return_an_array
    assert_kind_of Array, Topic.last(5)
  end

+  def test_take_and_first_and_last_with_limit_should_return_same_size
+    topics = Topic.limit(1)
+    assert_equal topics.first(2).size, topics.last(2).size


first with limit doesn't work as expected, and inconsistent with last is true.
But it is also inconsistent with loaded first with limit.
Can you more exercise tests like test_last_on_relation_with_limit_and_offset?

def test_find_on_relation_with_limit_and_offset post = posts("sti_comments") comments = post.comments.order(id: :asc) assert_equal comments.limit(2).to_a.first, comments.limit(2).first assert_equal comments.limit(2).to_a.first(2), comments.limit(2).first(2) assert_equal comments.limit(2).to_a.first(3), comments.limit(2).first(3) assert_equal comments.offset(2).to_a.first, comments.offset(2).first assert_equal comments.offset(2).to_a.first(2), comments.offset(2).first(2) assert_equal comments.offset(2).to_a.first(3), comments.offset(2).first(3) comments = comments.offset(1) assert_equal comments.limit(2).to_a.first, comments.limit(2).first assert_equal comments.limit(2).to_a.first(2), comments.limit(2).first(2) assert_equal comments.limit(2).to_a.first(3), comments.limit(2).first(3) end

rails/activerecord/test/cases/finder_test.rb

Lines 672 to 688 in 7c69351

def test_last_on_relation_with_limit_and_offset

post = posts("sti_comments")

comments = post.comments.order(id: :asc)

assert_equal comments.limit(2).to_a.last, comments.limit(2).last

assert_equal comments.limit(2).to_a.last(2), comments.limit(2).last(2)

assert_equal comments.limit(2).to_a.last(3), comments.limit(2).last(3)

assert_equal comments.offset(2).to_a.last, comments.offset(2).last

assert_equal comments.offset(2).to_a.last(2), comments.offset(2).last(2)

assert_equal comments.offset(2).to_a.last(3), comments.offset(2).last(3)

comments = comments.offset(1)

assert_equal comments.limit(2).to_a.last, comments.limit(2).last

assert_equal comments.limit(2).to_a.last(2), comments.limit(2).last(2)

assert_equal comments.limit(2).to_a.last(3), comments.limit(2).last(3)

end

No problem, I have added additional tests for compatibility here between limit(x).first(y) and limit(x).to_a.first(y), and they all now pass as well.

brchristian · 2018-01-08T23:09:13Z

@kamipo Thanks for code review. I have refactored and added the additional tests.

kamipo · 2018-01-09T13:04:59Z

activerecord/test/cases/finder_test.rb

+
+    assert_equal topics.limit(1).to_a.first(2).size, topics.limit(1).first(2).size
+    assert_equal topics.limit(2).to_a.first(2).size, topics.limit(2).first(2).size
+    assert_equal topics.limit(3).to_a.first(2).size, topics.limit(3).first(2).size


Can you remove .size? These need to be the same result regardless of whether they are loaded or not.

kamipo · 2018-01-09T13:08:57Z

activerecord/test/cases/finder_test.rb

+
+    assert_equal topics.limit(1).to_a.last(2).size, topics.limit(1).last(2).size
+    assert_equal topics.limit(2).to_a.last(2).size, topics.limit(2).last(2).size
+    assert_equal topics.limit(3).to_a.last(2).size, topics.limit(3).last(2).size


These assersions have already been covered by test_last_on_relation_with_limit_and_offset.

rails/activerecord/test/cases/finder_test.rb

Lines 672 to 688 in 7c69351

def test_last_on_relation_with_limit_and_offset

post = posts("sti_comments")

comments = post.comments.order(id: :asc)

assert_equal comments.limit(2).to_a.last, comments.limit(2).last

assert_equal comments.limit(2).to_a.last(2), comments.limit(2).last(2)

assert_equal comments.limit(2).to_a.last(3), comments.limit(2).last(3)

assert_equal comments.offset(2).to_a.last, comments.offset(2).last

assert_equal comments.offset(2).to_a.last(2), comments.offset(2).last(2)

assert_equal comments.offset(2).to_a.last(3), comments.offset(2).last(3)

comments = comments.offset(1)

assert_equal comments.limit(2).to_a.last, comments.limit(2).last

assert_equal comments.limit(2).to_a.last(2), comments.limit(2).last(2)

assert_equal comments.limit(2).to_a.last(3), comments.limit(2).last(3)

end

kamipo · 2018-01-09T13:24:01Z

activerecord/CHANGELOG.md

@@ -1,3 +1,11 @@
+*   Fixed inconsistency between `first(n)` and `last(n)` when
+    used with `limit()` The `first` finder respects the `limit()`,
+    making it consistent with `last(n)`.


I think that we should note fixing inconsistency first(n) with either loaded or not.
Previously, we need to use relation.to_a.first(n) as a workaround, but now, we can use relation.first(n) simply.

kamipo · 2018-01-09T13:34:47Z

activerecord/test/cases/finder_test.rb

+
+    assert_equal topics.limit(1).to_a.first(2).size, topics.limit(1).to_a.last(2).size
+    assert_equal topics.limit(2).to_a.first(2).size, topics.limit(2).to_a.last(2).size
+    assert_equal topics.limit(3).to_a.first(2).size, topics.limit(3).to_a.last(2).size


Array#first and Array#last are in the Ruby Core. These assertions are not necessary.

http://ruby-doc.org/core-2.5.0/Array.html#method-i-first
http://ruby-doc.org/core-2.5.0/Array.html#method-i-last

kamipo · 2018-01-09T14:03:19Z

activerecord/test/cases/finder_test.rb

+
+    assert_equal topics.limit(1).first(2).size, topics.limit(1).last(2).size
+    assert_equal topics.limit(2).first(2).size, topics.limit(2).last(2).size
+    assert_equal topics.limit(3).first(2).size, topics.limit(3).last(2).size


Previously relation.to_a.first(n), relation.to_a.last(n), and relation.last(n) works expected, but only relation.first(n) does not.
Consistency between relation.to_a.last(n) and relation.last(n) have already been covered by test_last_on_relation_with_limit_and_offset.
Whether relation.first(n) works correctly should compare relation.to_a.first(n) rather than relation.last(n).

brchristian · 2018-01-09T19:15:45Z

@kamipo Great points about the tests and CHANGELOG. I agree with you and I have made all the changes you suggested.

kamipo

Looks good to me 👍
Can you squash your commits into one?

brchristian · 2018-01-09T19:29:33Z

@kamipo Squashed 👍

brchristian · 2018-01-09T20:12:47Z

(PS, Special thanks to @mmlindeboom for helpful discussion!)

rails-bot assigned pixeltrix Jan 6, 2017

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brchristian mentioned this pull request Jan 6, 2017

ActiveRecord find_nth finder respects limit() #24124

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kamipo mentioned this pull request Jan 7, 2018

Fix ActiveRecord's 'first' method with 'limit'. Fixes #23979 #24060

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resolve inconsistencies between first and to_a.first with limit

b75a67c

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ActiveRecord: limit() and primary_key #24131

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Consistency between first() and last() with limit #27597

Consistency between first() and last() with limit #27597

brchristian commented Jan 6, 2017 •

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brchristian commented Jan 9, 2018

	def test_last_on_relation_with_limit_and_offset
	post = posts("sti_comments")

	comments = post.comments.order(id: :asc)
	assert_equal comments.limit(2).to_a.last, comments.limit(2).last
	assert_equal comments.limit(2).to_a.last(2), comments.limit(2).last(2)
	assert_equal comments.limit(2).to_a.last(3), comments.limit(2).last(3)

	assert_equal comments.offset(2).to_a.last, comments.offset(2).last
	assert_equal comments.offset(2).to_a.last(2), comments.offset(2).last(2)
	assert_equal comments.offset(2).to_a.last(3), comments.offset(2).last(3)

	comments = comments.offset(1)
	assert_equal comments.limit(2).to_a.last, comments.limit(2).last
	assert_equal comments.limit(2).to_a.last(2), comments.limit(2).last(2)
	assert_equal comments.limit(2).to_a.last(3), comments.limit(2).last(3)
	end

Consistency between first() and last() with limit #27597

Consistency between first() and last() with limit #27597

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