Fix `first(limit)` to take advantage of `loaded?` records if available

I realized that `first(2)`, etc. was unnecessarily querying for the
records when they were already preloaded. This was because
`find_nth_with_limit` can not know which `@records` to return because
it conflates the `offset` and `index` into a single variable, while
the `@records` only needs the `index` itself to select the proper
record.

Because `find_nth` and `find_nth_with_limit` are public methods, I
instead introduced a private method `find_nth_with_limit_and_offset`
which is called internally and handles the `loaded?` checking.

Once the `offset` argument is removed from `find_nth`,
`find_nth_with_limit_and_offset` can be collapsed into
`find_nth_with_limit`, with `offset` always equal to `offset_index`.

1 files changed, 14 insertions, 3 deletions

diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 7149c7d072..df0c2f1f92 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -111,10 +111,21 @@ class RelationTest < ActiveRecord::TestCase
 
   def test_loaded_first
     topics = Topic.all.order('id ASC')
+    topics.to_a # force load
 
-    assert_queries(1) do
-      topics.to_a # force load
-      2.times { assert_equal "The First Topic", topics.first.title }
+    assert_no_queries do
+      assert_equal "The First Topic", topics.first.title
+    end
+
+    assert topics.loaded?
+  end
+
+  def test_loaded_first_with_limit
+    topics = Topic.all.order('id ASC')
+    topics.to_a # force load
+
+    assert_no_queries do
+      assert_equal "The First Topic", topics.first(2).first.title
     end
 
     assert topics.loaded?