Ensure `has_and_belongs_to_many` works with `belongs_to_required_by_default`

Before this commit, if
`ActiveRecord::Base.belongs_to_required_by_default` is set to `true`,
then creating a record through `has_and_belongs_to_many` fails with the
cryptic error message `Left side must exist`. This is because
`inverse_of` isn't working properly in this case, presumably since we're
doing trickery with anonymous classes in the middle.

Rather than following this rabbit hole to try and get `inverse_of` to
work in a case that we know is not publicly supported, we can just turn
off this validation to match the behavior of 4.2 and earlier.

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb b/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
index e9f679e6de..ccb062f991 100644
--- a/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
@@ -976,4 +976,10 @@ class HasAndBelongsToManyAssociationsTest < ActiveRecord::TestCase
     assert preloaded_first_project.salaried_developers.loaded?, true
     assert_equal first_project.salaried_developers.size, preloaded_first_project.salaried_developers.size
   end
+
+  def test_has_and_belongs_to_many_is_useable_with_belongs_to_required_by_default
+    assert_difference "Project.first.developers_required_by_default.size", 1 do
+      Project.first.developers_required_by_default.create!(name: "Sean", salary: 50000)
+    end
+  end
 end