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.

activerecord/lib/active_record/associations.rb

@@ -1309,6 +1309,10 @@ def has_many(name, scope = nil, options = {}, &extension)
       #   that is the inverse of this <tt>has_one</tt> association. Does not work in combination
       #   with <tt>:through</tt> or <tt>:as</tt> options.
       #   See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.
+      # [:required]
+      #   When set to +true+, the association will also have its presence validated.
+      #   This will validate the association itself, not the id. You can use
+      #   +:inverse_of+ to avoid an extra query during validation.
       #
       # Option examples:
       #   has_one :credit_card, dependent: :destroy  # destroys the associated credit card
@@ -1320,6 +1324,7 @@ def has_many(name, scope = nil, options = {}, &extension)
       #   has_one :boss, readonly: :true
       #   has_one :club, through: :membership
       #   has_one :primary_address, -> { where primary: true }, through: :addressables, source: :addressable
+      #   has_one :credit_card, required: true
       def has_one(name, scope = nil, options = {})
         reflection = Builder::HasOne.build(self, name, scope, options)
         Reflection.add_reflection self, name, reflection
@@ -1421,6 +1426,10 @@ def has_one(name, scope = nil, options = {})
       #   object that is the inverse of this <tt>belongs_to</tt> association. Does not work in
       #   combination with the <tt>:polymorphic</tt> options.
       #   See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.
+      # [:required]
+      #   When set to +true+, the association will also have its presence validated.
+      #   This will validate the association itself, not the id. You can use
+      #   +:inverse_of+ to avoid an extra query during validation.
       #
       # Option examples:
       #   belongs_to :firm, foreign_key: "client_of"
@@ -1433,6 +1442,7 @@ def has_one(name, scope = nil, options = {})
       #   belongs_to :post, counter_cache: true
       #   belongs_to :company, touch: true
       #   belongs_to :company, touch: :employees_last_updated_at
+      #   belongs_to :company, required: true
       def belongs_to(name, scope = nil, options = {})
         reflection = Builder::BelongsTo.build(self, name, scope, options)
         Reflection.add_reflection self, name, reflection

activerecord/lib/active_record/associations/builder/association.rb

@@ -36,6 +36,7 @@ def self.build(model, name, scope, options, &block)
       reflection = builder.build(model)
       define_accessors model, reflection
       define_callbacks model, reflection
+      define_validations model, reflection
       builder.define_extensions model
       reflection
     end
@@ -124,6 +125,10 @@ def #{name}=(value)
       CODE
     end
 
+    def self.define_validations(model, reflection)
+      # noop
+    end
+
     def self.valid_dependent_options
       raise NotImplementedError
     end

activerecord/lib/active_record/associations/builder/singular_association.rb

@@ -3,7 +3,7 @@
 module ActiveRecord::Associations::Builder
   class SingularAssociation < Association #:nodoc:
     def valid_options
-      super + [:remote, :dependent, :primary_key, :inverse_of]
+      super + [:remote, :dependent, :primary_key, :inverse_of, :required]
     end
 
     def self.define_accessors(model, reflection)
@@ -27,5 +27,12 @@ def create_#{name}!(*args, &block)
         end
       CODE
     end
+
+    def self.define_validations(model, reflection)
+      super
+      if reflection.options[:required]
+        model.validates_presence_of reflection.name
+      end
+    end
   end
 end

activerecord/test/cases/associations/required_test.rb

+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