allow optional metadata to be omitted as well as nulled out.

Author: mattpolzin

Sources/JSONAPI/Resource/Relationship.swift

@@ -263,9 +263,6 @@ extension MetaRelationship: Codable {
     }
 }
 
-fileprivate protocol _Optional {}
-extension Optional: _Optional {}
-
 extension ToOneRelationship: Codable where Identifiable.ID: OptionalId {
     public init(from decoder: Decoder) throws {
         let container = try decoder.container(keyedBy: ResourceLinkageCodingKeys.self)

Sources/JSONAPI/Resource/Resource Object/ResourceObject.swift

@@ -447,7 +447,17 @@ public extension ResourceObject {
             )
         }
 
-        meta = try (NoMetadata() as? MetaType) ?? container.decode(MetaType.self, forKey: .meta)
+        do {
+            meta = try (NoMetadata() as? MetaType) ?? container.decode(MetaType.self, forKey: .meta)
+        } catch let decodingError as DecodingError {
+            let anyNil: Any? = nil
+
+            guard case .keyNotFound = decodingError,
+                  let omittedMeta = anyNil as? MetaType else {
+                throw decodingError
+            }
+            meta = omittedMeta
+        }
 
         links = try (NoLinks() as? LinksType) ?? container.decode(LinksType.self, forKey: .links)
     }

Tests/JSONAPITests/ResourceObject/ResourceObjectTests.swift

@@ -151,6 +151,11 @@ extension ResourceObjectTests {
 								   data: entity_no_relationships_no_attributes)
 	}
 
+  func test_EntityNoMetaDecodesAsOptionalMeta() {
+      let _ = decoded(type: TestEntityOptionalMeta.self,
+                      data: entity_no_relationships_no_attributes)
+  }
+
 	func test_EntityNoRelationshipsSomeAttributes() {
 		let entity = decoded(type: TestEntity5.self,
 								   data: entity_no_relationships_some_attributes)
@@ -861,13 +866,13 @@ extension ResourceObjectTests {
 
 		public struct Relationships: JSONAPI.Relationships {
 			public init() {
-                optionalMeta = nil
+        optionalMeta = nil
 				optionalOne = nil
 				optionalNullableOne = nil
 				optionalMany = nil
 			}
 
-            let optionalMeta: MetaRelationship<TestEntityMeta, NoLinks>?
+      let optionalMeta: MetaRelationship<TestEntityMeta, NoLinks>?
 
 			let optionalOne: ToOneRelationship<TestEntity1, NoIdMetadata, NoMetadata, NoLinks>?
 
@@ -879,6 +884,19 @@ extension ResourceObjectTests {
 
 	typealias TestEntity12 = BasicEntity<TestEntityType12>
 
+  enum TestEntityOptionalMetaType: JSONAPI.ResourceObjectDescription {
+      public static var jsonType: String { return "test_entities" }
+
+      typealias Attributes = NoAttributes
+      typealias Relationships = NoRelationships
+  }
+
+  struct UnimportantMeta: JSONAPI.Meta {
+      let property1: String
+  }
+
+  typealias TestEntityOptionalMeta = JSONAPI.ResourceObject<TestEntityOptionalMetaType, UnimportantMeta?, NoLinks, String>
+
 	enum UnidentifiedTestEntityType: ResourceObjectDescription {
 		public static var jsonType: String { return "unidentified_test_entities" }