Add :readonly option to attributes and relationships

This commit adds the :readonly flag to both the attribute method and the
relationship method on JSONAPI::Resource. Flagging an attribute or
relationship as readonly prevents it from being included in either the
`creatable_fields` or `updatable_fields`.

Author: hidde-jan

lib/jsonapi/relationship.rb

@@ -60,6 +60,10 @@ def belongs_to?
       false
     end
 
+    def readonly?
+      @options[:readonly]
+    end
+
     class ToOne < Relationship
       attr_reader :foreign_key_on
 

lib/jsonapi/resource.rb

@@ -580,12 +580,12 @@ def cache_field(field)
 
       # Override in your resource to filter the updatable keys
       def updatable_fields(_context = nil)
-        _updatable_relationships | _attributes.keys - [:id]
+        _updatable_relationships | _updatable_attributes - [:id]
       end
 
       # Override in your resource to filter the creatable keys
       def creatable_fields(_context = nil)
-        _updatable_relationships | _attributes.keys
+        _updatable_relationships | _updatable_attributes
       end
 
       # Override in your resource to filter the sortable keys
@@ -891,8 +891,12 @@ def _attribute_options(attr)
         default_attribute_options.merge(@_attributes[attr])
       end
 
+      def _updatable_attributes
+        _attributes.map { |key, options| key unless options[:readonly] }.compact
+      end
+
       def _updatable_relationships
-        @_relationships.map { |key, _relationship| key }
+        @_relationships.map { |key, relationship| key unless relationship.readonly? }.compact
       end
 
       def _relationship(type)

test/unit/resource/resource_test.rb

@@ -98,6 +98,11 @@ class RelatedResource < MyModule::RelatedResource
   end
 end
 
+class PostWithReadonlyAttributesResource < JSONAPI::Resource
+  attribute :title, readonly: true
+  has_one :author, readonly: true
+end
+
 class ResourceTest < ActiveSupport::TestCase
   def setup
     @post = Post.first
@@ -629,4 +634,12 @@ def test_resources_for_transforms_records_into_resources
     resources = PostResource.resources_for([Post.first], {})
     assert_equal(PostResource, resources.first.class)
   end
+
+  def test_readonly_attribute
+    refute_includes(PostWithReadonlyAttributesResource.creatable_fields, :title)
+    refute_includes(PostWithReadonlyAttributesResource.updatable_fields, :title)
+
+    refute_includes(PostWithReadonlyAttributesResource.creatable_fields, :author)
+    refute_includes(PostWithReadonlyAttributesResource.updatable_fields, :author)
+  end
 end