Consistency between first() and last() with limit

[brchristian]

Fixes #23979.

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]

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[kamipo]

Can you rebase?

[brchristian]

@kamipo Rebased to latest master 👍

[kamipo]

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

[brchristian]

Great point! I have updated the PR.

[kamipo]

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

[brchristian]

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]

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

[kamipo]

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

[kamipo]

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]

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]

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]

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]

@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]

@kamipo Squashed 👍

[brchristian]

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