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.

3 files changed, 23 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 6222bfe903..d9b339a1f6 100644
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -1309,6 +1309,10 @@ module ActiveRecord
       #   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 @@ module ActiveRecord
       #   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 @@ module ActiveRecord
       #   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 @@ module ActiveRecord
       #   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
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