Add a `required` option to singular associations

In addition to defining the association, a `required` association will
also have its presence validated.

Before:

```ruby
belongs_to :account
validates_presence_of :account
```

After:

```ruby
belongs_to :account, required: true
```

This helps to draw a distinction between types of validations, since
validations on associations are generally for data integrity purposes,
and aren't usually set through form inputs.

1 files changed, 82 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/required_test.rb b/activerecord/test/cases/associations/required_test.rb
new file mode 100644
index 0000000000..a6934a056e
--- /dev/null
+++ b/activerecord/test/cases/associations/required_test.rb
@@ -0,0 +1,82 @@
+require "cases/helper"
+
+class RequiredAssociationsTest < ActiveRecord::TestCase
+  self.use_transactional_fixtures = false
+
+  class Parent < ActiveRecord::Base
+  end
+
+  class Child < ActiveRecord::Base
+  end
+
+  setup do
+    @connection = ActiveRecord::Base.connection
+    @connection.create_table :parents, force: true
+    @connection.create_table :children, force: true do |t|
+      t.belongs_to :parent
+    end
+  end
+
+  teardown do
+    @connection.execute("DROP TABLE IF EXISTS parents")
+    @connection.execute("DROP TABLE IF EXISTS children")
+  end
+
+  test "belongs_to associations are not required by default" do
+    model = subclass_of(Child) do
+      belongs_to :parent, inverse_of: false,
+        class_name: "RequiredAssociationsTest::Parent"
+    end
+
+    assert model.new.save
+    assert model.new(parent: Parent.new).save
+  end
+
+  test "required belongs_to associations have presence validated" do
+    model = subclass_of(Child) do
+      belongs_to :parent, required: true, inverse_of: false,
+        class_name: "RequiredAssociationsTest::Parent"
+    end
+
+    record = model.new
+    assert_not record.save
+    assert_equal ["Parent can't be blank"], record.errors.full_messages
+
+    record.parent = Parent.new
+    assert record.save
+  end
+
+  test "has_one associations are not required by default" do
+    model = subclass_of(Parent) do
+      has_one :child, inverse_of: false,
+        class_name: "RequiredAssociationsTest::Child"
+    end
+
+    assert model.new.save
+    assert model.new(child: Child.new).save
+  end
+
+  test "required has_one associations have presence validated" do
+    model = subclass_of(Parent) do
+      has_one :child, required: true, inverse_of: false,
+        class_name: "RequiredAssociationsTest::Child"
+    end
+
+    record = model.new
+    assert_not record.save
+    assert_equal ["Child can't be blank"], record.errors.full_messages
+
+    record.child = Child.new
+    assert record.save
+  end
+
+  private
+
+  def subclass_of(klass, &block)
+    subclass = Class.new(klass, &block)
+    def subclass.name
+      superclass.name
+    end
+    subclass
+  end
+end