fix(Deserializer): deserialize indpendent hasone pointers

src/JsonApiDotNetCore/Models/ResourceIdentifierObject.cs

@@ -4,9 +4,16 @@ namespace JsonApiDotNetCore.Models
 {
     public class ResourceIdentifierObject
     {
+        public ResourceIdentifierObject() { }
+        public ResourceIdentifierObject(string type, string id)
+        {
+            Type = type;
+            Id = id;
+        }
+
         [JsonProperty("type")]
         public string Type { get; set; }
-        
+
         [JsonProperty("id")]
         public string Id { get; set; }
 

src/JsonApiDotNetCore/Serialization/JsonApiDeSerializer.cs

@@ -204,53 +204,59 @@ private object SetHasOneRelationship(object entity,
             if (relationships.TryGetValue(relationshipName, out RelationshipData relationshipData) == false)
                 return entity;
 
-            var relationshipAttr = _jsonApiContext.RequestEntity.Relationships
-                .SingleOrDefault(r => r.PublicRelationshipName == relationshipName);
-
-            if (relationshipAttr == null)
-                throw new JsonApiException(400, $"{_jsonApiContext.RequestEntity.EntityName} does not contain a relationship '{relationshipName}'");
-
             var rio = (ResourceIdentifierObject)relationshipData.ExposedData;
 
             var foreignKey = attr.IdentifiablePropertyName;
             var foreignKeyProperty = entityProperties.FirstOrDefault(p => p.Name == foreignKey);
-
             if (foreignKeyProperty == null && rio == null)
                 return entity;
 
-            if (foreignKeyProperty == null && rio != null)
-                throw new JsonApiException(400, $"{contextEntity.EntityType.Name} does not contain a foreign key property '{foreignKey}' for has one relationship '{attr.InternalRelationshipName}'");
-
-            // e.g. PATCH /articles
-            // {... { "relationships":{ "Owner": { "data": null } } } }
-            if (rio == null && Nullable.GetUnderlyingType(foreignKeyProperty.PropertyType) == null)
-                throw new JsonApiException(400, $"Cannot set required relationship identifier '{attr.IdentifiablePropertyName}' to null because it is a non-nullable type.");
+            SetHasOneForeignKeyValue(entity, attr, foreignKeyProperty, rio);
+            SetHasOneNavigationPropertyValue(entity, attr, rio, included);
 
-            var newValue = rio?.Id ?? null;
-            var convertedValue = TypeHelper.ConvertType(newValue, foreignKeyProperty.PropertyType);
+            return entity;
+        }
 
-            _jsonApiContext.RelationshipsToUpdate[relationshipAttr] = convertedValue;
+        private void SetHasOneForeignKeyValue(object entity, HasOneAttribute hasOneAttr, PropertyInfo foreignKeyProperty, ResourceIdentifierObject rio)
+        {
+            var foreignKeyPropertyValue = rio?.Id ?? null;
+            if (foreignKeyProperty != null)
+            {
+                // in the case of the HasOne independent side of the relationship, we should still create the shell entity on the other side
+                // we should not actually require the resource to have a foreign key (be the dependent side of the relationship)
 
-            foreignKeyProperty.SetValue(entity, convertedValue);
+                // e.g. PATCH /articles
+                // {... { "relationships":{ "Owner": { "data": null } } } }
+                if (rio == null && Nullable.GetUnderlyingType(foreignKeyProperty.PropertyType) == null)
+                    throw new JsonApiException(400, $"Cannot set required relationship identifier '{hasOneAttr.IdentifiablePropertyName}' to null because it is a non-nullable type.");
 
+                var convertedValue = TypeHelper.ConvertType(foreignKeyPropertyValue, foreignKeyProperty.PropertyType);
+                foreignKeyProperty.SetValue(entity, convertedValue);
+                _jsonApiContext.RelationshipsToUpdate[hasOneAttr] = convertedValue;
+            }
+        }
 
-            if (rio != null
-                // if the resource identifier is null, there should be no reason to instantiate an instance
-                && rio.Id != null)
+        /// <summary>
+        /// Sets the value of the navigation property for the related resource.
+        /// If the resource has been included, all attributes will be set.
+        /// If the resource has not been included, only the id will be set.
+        /// </summary>
+        private void SetHasOneNavigationPropertyValue(object entity, HasOneAttribute hasOneAttr, ResourceIdentifierObject rio, List<DocumentData> included)
+        {
+            // if the resource identifier is null, there should be no reason to instantiate an instance
+            if (rio != null && rio.Id != null)
             {
                 // we have now set the FK property on the resource, now we need to check to see if the
                 // related entity was included in the payload and update its attributes
-                var includedRelationshipObject = GetIncludedRelationship(rio, included, relationshipAttr);
+                var includedRelationshipObject = GetIncludedRelationship(rio, included, hasOneAttr);
                 if (includedRelationshipObject != null)
-                    relationshipAttr.SetValue(entity, includedRelationshipObject);
+                    hasOneAttr.SetValue(entity, includedRelationshipObject);
 
                 // we need to store the fact that this relationship was included in the payload
                 // for EF, the repository will use these pointers to make ensure we don't try to
                 // create resources if they already exist, we just need to create the relationship
-                _jsonApiContext.HasOneRelationshipPointers.Add(attr, includedRelationshipObject);
+                _jsonApiContext.HasOneRelationshipPointers.Add(hasOneAttr, includedRelationshipObject);
             }
-
-            return entity;
         }
 
         private object SetHasManyRelationship(object entity,

test/UnitTests/Serialization/JsonApiDeSerializerTests.cs

@@ -240,6 +240,7 @@ public void Can_Deserialize_Independent_Side_Of_One_To_One_Relationship_With_Rel
             jsonApiContextMock.SetupAllProperties();
             jsonApiContextMock.Setup(m => m.ContextGraph).Returns(contextGraph);
             jsonApiContextMock.Setup(m => m.AttributesToUpdate).Returns(new Dictionary<AttrAttribute, object>());
+            jsonApiContextMock.Setup(m => m.HasOneRelationshipPointers).Returns(new HasOneRelationshipPointers());
 
             var jsonApiOptions = new JsonApiOptions();
             jsonApiContextMock.Setup(m => m.Options).Returns(jsonApiOptions);
@@ -255,7 +256,14 @@ public void Can_Deserialize_Independent_Side_Of_One_To_One_Relationship_With_Rel
                     Id = "1",
                     Attributes = new Dictionary<string, object> { { "property", property } },
                     // a common case for this is deserialization in unit tests
-                    Relationships = new Dictionary<string, RelationshipData> { { "dependent", new RelationshipData { } } }
+                    Relationships = new Dictionary<string, RelationshipData> {
+                        {
+                            "dependent", new RelationshipData
+                            {
+                                SingleData = new ResourceIdentifierObject("dependents", "1")
+                            }
+                        }
+                    }
                 }
             };
 
@@ -267,6 +275,8 @@ public void Can_Deserialize_Independent_Side_Of_One_To_One_Relationship_With_Rel
             // assert
             Assert.NotNull(result);
             Assert.Equal(property, result.Property);
+            Assert.NotNull(result.Dependent);
+            Assert.Equal(1, result.Dependent.Id);
         }
 
         [Fact]