Fix issue with collection associations and first(n)/last(n)

When calling first(n) or last(n) in a collection, Active Record was
improperly trying to set the inverse of instance in case that option
existed. This change was introduced by
fdf4eae506fa9895e831f569bed3c4aa6a999a22.

In such cases we don't need to do that "manually", since the way
collection will be loaded will already handle that, so we just skip
setting the inverse association when any argument is given to
first(n)/last(n).

The test included ensures that these scenarios will have the inverse of
instance set properly.

Fixes #8087, Closes #8094.

Squashed cherry-pick from d37d40b and c368b66.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/associations/collection_association.rb

2 files changed, 16 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index 7f39d3083e..54215cf88d 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -576,7 +576,9 @@ module ActiveRecord
           args.shift if args.first.is_a?(Hash) && args.first.empty?
 
           collection = fetch_first_or_last_using_find?(args) ? scope : load_target
-          collection.send(type, *args).tap {|it| set_inverse_instance it }
+          collection.send(type, *args).tap do |record|
+            set_inverse_instance record if record.is_a? ActiveRecord::Base
+          end
         end
     end
   end
diff --git a/activerecord/test/cases/associations/inverse_associations_test.rb b/activerecord/test/cases/associations/inverse_associations_test.rb
index aad48e7ce9..8c9b4fb921 100644
--- a/activerecord/test/cases/associations/inverse_associations_test.rb
+++ b/activerecord/test/cases/associations/inverse_associations_test.rb
@@ -261,10 +261,23 @@ class InverseHasManyTests < ActiveRecord::TestCase
 
   def test_parent_instance_should_be_shared_with_first_and_last_child
     man = Man.first
+
     assert man.interests.first.man.equal? man
     assert man.interests.last.man.equal? man
   end
 
+  def test_parent_instance_should_be_shared_with_first_n_and_last_n_children
+    man = Man.first
+
+    interests = man.interests.first(2)
+    assert interests[0].man.equal? man
+    assert interests[1].man.equal? man
+
+    interests = man.interests.last(2)
+    assert interests[0].man.equal? man
+    assert interests[1].man.equal? man
+  end
+
   def test_trying_to_use_inverses_that_dont_exist_should_raise_an_error
     assert_raise(ActiveRecord::InverseOfAssociationNotFoundError) { Man.first.secret_interests }
   end