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.

3 files changed, 16 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
 
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
diff --git a/activerecord/test/models/project.rb b/activerecord/test/models/project.rb
index b034e0e267..efa8246f1e 100644
--- a/activerecord/test/models/project.rb
+++ b/activerecord/test/models/project.rb
@@ -15,6 +15,14 @@ class Project < ActiveRecord::Base
   belongs_to :firm
   has_one :lead_developer, through: :firm, inverse_of: :contracted_projects
 
+  begin
+    previous_value, ActiveRecord::Base.belongs_to_required_by_default =
+      ActiveRecord::Base.belongs_to_required_by_default, true
+    has_and_belongs_to_many :developers_required_by_default, class_name: "Developer"
+  ensure
+    ActiveRecord::Base.belongs_to_required_by_default = previous_value
+  end
+
   attr_accessor :developers_log
   after_initialize :set_developers_log