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.

2 files changed, 13 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/builder/association.rb b/activerecord/lib/active_record/associations/builder/association.rb
index e474236939..947d61ee7b 100644
--- a/activerecord/lib/active_record/associations/builder/association.rb
+++ b/activerecord/lib/active_record/associations/builder/association.rb
@@ -36,6 +36,7 @@ module ActiveRecord::Associations::Builder
       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 @@ module ActiveRecord::Associations::Builder
       CODE
     end
 
+    def self.define_validations(model, reflection)
+      # noop
+    end
+
     def self.valid_dependent_options
       raise NotImplementedError
     end
diff --git a/activerecord/lib/active_record/associations/builder/singular_association.rb b/activerecord/lib/active_record/associations/builder/singular_association.rb
index e655c389a6..17f36acf40 100644
--- a/activerecord/lib/active_record/associations/builder/singular_association.rb
+++ b/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 @@ module ActiveRecord::Associations::Builder
         end
       CODE
     end
+
+    def self.define_validations(model, reflection)
+      super
+      if reflection.options[:required]
+        model.validates_presence_of reflection.name
+      end
+    end
   end
 end