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, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb b/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
index a5c9f1666e..b888148841 100644
--- a/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
+++ b/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
@@ -62,13 +62,13 @@ module ActiveRecord::Associations::Builder # :nodoc:
         end
 
         def self.add_left_association(name, options)
-          belongs_to name, options
+          belongs_to name, required: false, **options
           self.left_reflection = _reflect_on_association(name)
         end
 
         def self.add_right_association(name, options)
           rhs_name = name.to_s.singularize.to_sym
-          belongs_to rhs_name, options
+          belongs_to rhs_name, required: false, **options
           self.right_reflection = _reflect_on_association(rhs_name)
         end