Add :default option to belongs_to (#28453)

Use it to specify that an association should be initialized with a
particular record before validation. For example:

    # Before
    belongs_to :account
    before_validation -> { self.account ||= Current.account }

    # After
    belongs_to :account, default: -> { Current.account }

1 files changed, 20 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index 5875a1871f..5b08ba1358 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -116,6 +116,26 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     ActiveRecord::Base.belongs_to_required_by_default = original_value
   end
 
+  def test_default
+    david = developers(:david)
+    jamis = developers(:jamis)
+
+    model = Class.new(ActiveRecord::Base) do
+      self.table_name = "ships"
+      def self.name; "Temp"; end
+      belongs_to :developer, default: -> { david }
+    end
+
+    ship = model.create!
+    assert_equal david, ship.developer
+
+    ship = model.create!(developer: jamis)
+    assert_equal jamis, ship.developer
+
+    ship.update!(developer: nil)
+    assert_equal david, ship.developer
+  end
+
   def test_default_scope_on_relations_is_not_cached
     counter = 0