Support implicit remove when creating/updating one-to-one relationship

src/Examples/JsonApiDotNetCoreExample/Data/AppDbContext.cs

@@ -50,6 +50,10 @@ protected override void OnModelCreating(ModelBuilder modelBuilder)
                 .WithOne(t => t.ParentTodoItem)
                 .HasForeignKey(t => t.ParentTodoItemId);
 
+            modelBuilder.Entity<Person>()
+                .HasOne(p => p.Passport)
+                .WithOne(p => p.Person)
+                .HasForeignKey<Person>(p => p.PassportId);
 
         }
 
@@ -61,13 +65,13 @@ protected override void OnModelCreating(ModelBuilder modelBuilder)
         public DbSet<Author> Authors { get; set; }
         public DbSet<NonJsonApiResource> NonJsonApiResources { get; set; }
         public DbSet<User> Users { get; set; }
-
         public DbSet<CourseEntity> Courses { get; set; }
         public DbSet<DepartmentEntity> Departments { get; set; }
         public DbSet<CourseStudentEntity> Registrations { get; set; }
         public DbSet<StudentEntity> Students { get; set; }
         public DbSet<PersonRole> PersonRoles { get; set; }
         public DbSet<ArticleTag> ArticleTags { get; set; }
         public DbSet<Tag> Tags { get; set; }
+        public DbSet<Passport> Passports { get; set; }
     }
 }

src/Examples/JsonApiDotNetCoreExample/Models/Passport.cs

@@ -0,0 +1,11 @@
+using JsonApiDotNetCore.Models;
+
+namespace JsonApiDotNetCoreExample.Models
+{
+    public class Passport : Identifiable
+    {
+        public virtual int? SocialSecurityNumber { get; set; }
+        [HasOne("person")]
+        public virtual Person Person { get; set; }
+    }
+}

src/Examples/JsonApiDotNetCoreExample/Models/Person.cs

@@ -38,6 +38,10 @@ public class Person : Identifiable, IHasMeta
         [HasOne("unincludeable-item", documentLinks: Link.All, canInclude: false)]
         public virtual TodoItem UnIncludeableItem { get; set; }
 
+        public int? PassportId { get; set; }
+        [HasOne("passport")]
+        public virtual Passport Passport { get; set; }
+
         public Dictionary<string, object> GetMeta(IJsonApiContext context)
         {
             return new Dictionary<string, object> {

src/JsonApiDotNetCore/Data/DefaultEntityRepository.cs

@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using System.Threading.Tasks;
 using JsonApiDotNetCore.Extensions;
 using JsonApiDotNetCore.Internal;
@@ -10,6 +11,7 @@
 using JsonApiDotNetCore.Models;
 using JsonApiDotNetCore.Services;
 using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata;
 using Microsoft.Extensions.Logging;
 
 namespace JsonApiDotNetCore.Data
@@ -338,6 +340,15 @@ public virtual async Task<TEntity> UpdateAsync(TId id, TEntity entity)
                 {
                     if ((relationship.Key.TypeId as Type).IsAssignableFrom(typeof(HasOneAttribute)))
                     {
+                        var attr = relationship.Key;
+                        if (_jsonApiContext.HasOneRelationshipPointers.Get().TryGetValue(attr, out var pointer) )
+                        {
+                            /// we need to attach inverse relations to make sure
+                            /// we're not violating any foreign key constraints 
+                            /// when implicitly removing pre-existing relations.
+                            /// See #502 for more info.
+                            _context.LoadInverseNavigation<TEntity>(attr, pointer);
+                        }
                         relationship.Key.SetValue(oldEntity, relationship.Value);
                     }
                     if ((relationship.Key.TypeId as Type).IsAssignableFrom(typeof(HasManyAttribute)))

src/JsonApiDotNetCore/Extensions/DbContextExtensions.cs

@@ -14,6 +14,39 @@ public static class DbContextExtensions
         public static DbSet<T> GetDbSet<T>(this DbContext context) where T : class 
             => context.Set<T>();
 
+        /// <summary>
+        /// Given a child entity and a relationship attribute between a parent 
+        /// entity to that child entity, attaches the entities on the inverse navigation
+        /// property to the dbContext.
+        /// </summary>
+        /// <param name="context">Context.</param>
+        /// <param name="parentToChildAttribute">Parent to child relationship attribute.</param>
+        /// <param name="childEntity">Child entity.</param>
+        /// <typeparam name="TParent">The 1st type parameter.</typeparam>
+        public static void LoadInverseNavigation<TParent>(
+            this DbContext context,
+            RelationshipAttribute parentToChildAttribute,
+            object childEntity) where TParent : class, IIdentifiable
+        {
+            var navigationMeta = context.Model
+                    .FindEntityType(typeof(TParent))
+                    .FindNavigation(parentToChildAttribute.InternalRelationshipName);
+            var inverseNavigationMeta = navigationMeta.FindInverse();
+            if (inverseNavigationMeta != null)
+            {
+                var inversePropertyType = inverseNavigationMeta.PropertyInfo.PropertyType;
+                var inversePropertyName = inverseNavigationMeta.Name;
+                var entityEntry = context.Entry(childEntity);
+                if (inversePropertyType.IsGenericType ) 
+                { // if generic, means we're dealing with a list
+                    entityEntry.Collection(inversePropertyName).Load();
+                } else
+                {
+                    entityEntry.Navigation(inversePropertyName).Load();
+                }
+            }
+        }
+
         /// <summary>
         /// Get the DbSet when the model type is unknown until runtime
         /// </summary>

test/JsonApiDotNetCoreExampleTests/Acceptance/Spec/UpdatingRelationshipsTests.cs

@@ -621,5 +621,57 @@ public async Task Can_Delete_Relationship_By_Patching_Relationship()
             Assert.Equal(HttpStatusCode.OK, response.StatusCode);
             Assert.Null(todoItemResult.Owner);
         }
+
+        [Fact]
+        public async Task Updating_ToOne_Relationship_With_Implicit_Remove()
+        {
+            // Arrange
+            var context = _fixture.GetService<AppDbContext>();
+            var passport = new Passport();
+            var person1 = _personFaker.Generate();
+            person1.Passport = passport;
+            var person2 = _personFaker.Generate();
+            context.People.AddRange(new List<Person>() { person1, person2 });
+            await context.SaveChangesAsync();
+
+            var passportId = person1.PassportId;
+
+            var content = new
+            {
+                data = new
+                {
+                    type = "people",
+                    id = person2.Id,
+                    relationships = new Dictionary<string, object>
+                    {
+                        { "passport", new
+                            {
+                                data = new { type = "passports", id = $"{passportId}" }
+                            }
+                        }
+                    }
+                }
+            };
+
+            var httpMethod = new HttpMethod("PATCH");
+            var route = $"/api/v1/people/{person2.Id}";
+            var request = new HttpRequestMessage(httpMethod, route);
+
+            string serializedContent = JsonConvert.SerializeObject(content);
+            request.Content = new StringContent(serializedContent);
+            request.Content.Headers.ContentType = new MediaTypeHeaderValue("application/vnd.api+json");
+
+            // Act
+            var response = await _fixture.Client.SendAsync(request);
+
+            // Assert
+            var body = await response.Content.ReadAsStringAsync();
+            Assert.True(HttpStatusCode.OK == response.StatusCode, $"{route} returned {response.StatusCode} status code with payload: {body}");
+
+            var _person1 = context.People.AsNoTracking().Include(ppl => ppl.Passport).Single(ppl => ppl.Id == person1.Id);
+            var _person2 = context.People.AsNoTracking().Include(ppl => ppl.Passport).Single(ppl => ppl.Id == person2.Id);
+            Assert.Null(_person1.Passport);
+            Assert.Equal(passportId, _person2.PassportId);
+        }
     }
 }