PolymorphicAssociationNullabilityChecker

PolymorphicAssociationNullabilityChecker finds polymorphic belongs_to associations whose foreign key and foreign type columns have different null constraints.

For an optional polymorphic association, both columns should allow NULL:

create_table :pictures do |t|
  t.bigint :imageable_id, null: true
  t.string :imageable_type, null: true
end

For a required polymorphic association, both columns should be NOT NULL. A mismatch allows an incomplete polymorphic reference where only the id or type is present.