Raise on `foreign_key:` being passed as an array in associations

Associations have never allowed nor supported `foreign_key` option
being passed as an Array. This still holds true for Rails 7.1
However with Rails 7.1 supporting composite primary keys it may become
more common for applications to mistakenly pass an array to `foreign_key:`.
This commit adds an exception to raise when `foreign_key:` is passed as
an array.

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Raise on `foreign_key:` being passed as an array in associations
+
+    *Nikita Vasilevsky*
+
 *   Ensure `#signed_id` outputs `url_safe` strings.
 
     *Jason Meller*

activerecord/lib/active_record/reflection.rb

@@ -382,6 +382,7 @@ module ActiveRecord
         @klass         = options[:anonymous_class]
         @plural_name   = active_record.pluralize_table_names ?
                             name.to_s.pluralize : name.to_s
+        validate_reflection!
       end
 
       def autosave=(autosave)
@@ -433,6 +434,17 @@ module ActiveRecord
         def derive_class_name
           name.to_s.camelize
         end
+
+        def validate_reflection!
+          return unless options[:foreign_key].is_a?(Array)
+
+          message = <<~MSG.squish
+            Passing #{options[:foreign_key]} array to :foreign_key option
+            on the #{active_record}##{name} association is not supported.
+            Use the query_constraints: #{options[:foreign_key]} option instead to represent a composite foreign key.
+          MSG
+          raise ArgumentError, message
+        end
     end
 
     # Holds all the metadata about an aggregation as it was specified in the

activerecord/test/cases/associations_test.rb

@@ -194,6 +194,30 @@ class AssociationsTest < ActiveRecord::TestCase
     assert_equal(expected_posts.map(&:id).sort, blog_posts.map(&:id).sort)
   end
 
+  def test_has_many_with_foreign_key_as_an_array_raises
+    expected_message = <<~MSG.squish
+      Passing [:blog_id, :blog_post_id] array to :foreign_key option
+      on the Sharded::BlogPost#broken_array_fk_comments association is not supported.
+      Use the query_constraints: [:blog_id, :blog_post_id] option instead to represent a composite foreign key.
+    MSG
+    assert_raises ArgumentError, match: expected_message do
+      Sharded::BlogPost.has_many :broken_array_fk_comments,
+        class_name: "Sharded::Comment", foreign_key: [:blog_id, :blog_post_id]
+    end
+  end
+
+  def test_belongs_to_with_foreign_key_as_an_array_raises
+    expected_message = <<~MSG.squish
+      Passing [:blog_id, :blog_post_id] array to :foreign_key option
+      on the Sharded::Comment#broken_array_fk_blog_post association is not supported.
+      Use the query_constraints: [:blog_id, :blog_post_id] option instead to represent a composite foreign key.
+    MSG
+    assert_raises ArgumentError, match: expected_message do
+      Sharded::Comment.belongs_to :broken_array_fk_blog_post,
+        class_name: "Sharded::Blog", foreign_key: [:blog_id, :blog_post_id]
+    end
+  end
+
   def test_has_many_association_with_composite_foreign_key_loads_records
     blog_post = sharded_blog_posts(:great_post_blog_one)
 

activerecord/test/cases/reflection_test.rb

@@ -226,6 +226,17 @@ class ReflectionTest < ActiveRecord::TestCase
     assert_equal "companies", Firm.reflect_on_association(:clients_of_firm).table_name
   end
 
+  def test_has_many_reflection_with_array_fk_raises
+    expected_message = <<~MSG.squish
+      Passing [:firm_id, :firm_name] array to :foreign_key option
+      on the Firm#clients association is not supported.
+      Use the query_constraints: [:firm_id, :firm_name] option instead to represent a composite foreign key.
+    MSG
+    assert_raises ArgumentError, match: expected_message do
+      ActiveRecord::Reflection.create(:has_many, :clients, nil, { foreign_key: [:firm_id, :firm_name] }, Firm)
+    end
+  end
+
   def test_has_one_reflection
     reflection_for_account = ActiveRecord::Reflection.create(:has_one, :account, nil, { foreign_key: "firm_id", dependent: :destroy }, Firm)
     assert_equal reflection_for_account, Firm.reflect_on_association(:account)
@@ -234,6 +245,17 @@ class ReflectionTest < ActiveRecord::TestCase
     assert_equal "accounts", Firm.reflect_on_association(:account).table_name
   end
 
+  def has_one_reflection_with_array_fk_raises
+    expected_message = <<~MSG.squish
+      Passing [:firm_id, :firm_name] array to :foreign_key option
+      on the Firm#account association is not supported.
+      Use the query_constraints: [:firm_id, :firm_name] option instead to represent a composite foreign key.
+    MSG
+    assert_raises ArgumentError, match: expected_message do
+      ActiveRecord::Reflection.create(:has_one, :account, nil, { foreign_key: [:firm_id, :firm_name] }, Firm)
+    end
+  end
+
   def test_belongs_to_inferred_foreign_key_from_assoc_name
     Company.belongs_to :foo
     assert_equal "foo_id", Company.reflect_on_association(:foo).foreign_key
@@ -243,6 +265,17 @@ class ReflectionTest < ActiveRecord::TestCase
     assert_equal "xyzzy_id", Company.reflect_on_association(:baz).foreign_key
   end
 
+  def test_belongs_to_reflection_with_array_fk_raises
+    expected_message = <<~MSG.squish
+      Passing [:firm_id, :firm_name] array to :foreign_key option
+      on the Firm#client association is not supported.
+      Use the query_constraints: [:firm_id, :firm_name] option instead to represent a composite foreign key.
+    MSG
+    assert_raises ArgumentError, match: expected_message do
+      ActiveRecord::Reflection.create(:belongs_to, :client, nil, { foreign_key: [:firm_id, :firm_name] }, Firm)
+    end
+  end
+
   def test_association_reflection_in_modules
     ActiveRecord::Base.store_full_sti_class = false