fix tests...lots of ugly code in here. need to refactor

Author: jaredcnance

src/JsonApiDotNetCore/Builders/ContextGraphBuilder.cs

@@ -192,9 +192,19 @@ protected virtual List<RelationshipAttribute> GetRelationships(Type entityType)
                     hasManyThroughAttribute.LeftProperty = throughProperties.SingleOrDefault(x => x.PropertyType == entityType)
                         ?? throw new JsonApiSetupException($"{hasManyThroughAttribute.ThroughType} does not contain a navigation property to type {entityType}");
 
+                    // ArticleTag.ArticleId
+                    var leftIdPropertyName = hasManyThroughAttribute.GetIdentityPropertyName(hasManyThroughAttribute.LeftProperty);
+                    hasManyThroughAttribute.LeftIdProperty = throughProperties.SingleOrDefault(p => p.Name == leftIdPropertyName)
+                        ?? throw new JsonApiSetupException($"{hasManyThroughAttribute.ThroughType} does not contain an identity property {leftIdPropertyName}");
+
                     // Article → ArticleTag.Tag
                     hasManyThroughAttribute.RightProperty = throughProperties.SingleOrDefault(x => x.PropertyType == hasManyThroughAttribute.Type)
                         ?? throw new JsonApiSetupException($"{hasManyThroughAttribute.ThroughType} does not contain a navigation property to type {hasManyThroughAttribute.Type}");
+                    
+                    // ArticleTag.TagId
+                    var rightIdPropertyName = hasManyThroughAttribute.GetIdentityPropertyName(hasManyThroughAttribute.RightProperty);
+                    hasManyThroughAttribute.RightIdProperty = throughProperties.SingleOrDefault(p => p.Name == rightIdPropertyName)
+                        ?? throw new JsonApiSetupException($"{hasManyThroughAttribute.ThroughType} does not contain an identity property {rightIdPropertyName}");
                 }
             }
 

src/JsonApiDotNetCore/Data/DefaultEntityRepository.cs

@@ -199,9 +199,7 @@ private void AttachHasManyPointers(TEntity entity)
         private void AttachHasMany(HasManyAttribute relationship, IList pointers)
         {
             foreach (var pointer in pointers)
-            {
                 _context.Entry(pointer).State = EntityState.Unchanged;
-            }
         }
 
         private void AttachHasManyThrough(TEntity entity, HasManyThroughAttribute hasManyThrough, IList pointers)
@@ -218,14 +216,48 @@ private void AttachHasManyThrough(TEntity entity, HasManyThroughAttribute hasMan
                 _context.Entry(pointer).State = EntityState.Unchanged;
 
                 var throughInstance = Activator.CreateInstance(hasManyThrough.ThroughType);
-                _context.Entry(throughInstance).State = EntityState.Added;
-
                 hasManyThrough.LeftProperty.SetValue(throughInstance, entity);
                 hasManyThrough.RightProperty.SetValue(throughInstance, pointer);
+
                 throughRelationshipCollection.Add(throughInstance);
             }
         }
 
+        private void UpdateHasManyThrough(TEntity entity)
+        {
+            var relationships = _jsonApiContext.HasManyRelationshipPointers.Get();
+            foreach (var relationship in relationships)
+            {
+                if(relationship.Key is HasManyThroughAttribute hasManyThrough)
+                {   
+                    // create the collection (e.g. List<ArticleTag>)
+                    // this type MUST implement IList so we can build the collection
+                    // if this is problematic, we _could_ reflect on the type and find an Add method
+                    // or we might be able to create a proxy type and implement the enumerator
+                    var throughRelationshipCollection = Activator.CreateInstance(hasManyThrough.ThroughProperty.PropertyType) as IList;
+                    hasManyThrough.ThroughProperty.SetValue(entity, throughRelationshipCollection);
+                    foreach (var pointer in relationship.Value)
+                    {                
+                        _context.Entry(pointer).State = EntityState.Unchanged;
+
+                        var throughInstance = Activator.CreateInstance(hasManyThrough.ThroughType);
+                        _context.Entry(throughInstance).State = EntityState.Added;
+
+                        hasManyThrough.LeftIdProperty.SetValue(throughInstance, entity.Id);
+                        hasManyThrough.LeftProperty.SetValue(throughInstance, entity);
+                        hasManyThrough.RightProperty.SetValue(throughInstance, pointer);
+                        
+                        var pointerId = (pointer as Identifiable<TId>) // known limitation, just need to come up with a solution...
+                            ?? throw new JsonApiException(500, $"Cannot update the HasManyThrough relationship '{hasManyThrough.PublicRelationshipName}'. Id type must match the parent resource id type.");
+
+                        hasManyThrough.RightIdProperty.SetValue(throughInstance, pointerId.Id);
+
+                        throughRelationshipCollection.Add(throughInstance);
+                    }
+                }
+            }
+        }
+
         /// <summary>
         /// This is used to allow creation of HasOne relationships when the
         /// independent side of the relationship already exists.
@@ -252,7 +284,7 @@ public virtual async Task<TEntity> UpdateAsync(TId id, TEntity entity)
             foreach (var relationship in _jsonApiContext.RelationshipsToUpdate)
                 relationship.Key.SetValue(oldEntity, relationship.Value);
 
-            AttachRelationships(entity);
+            UpdateHasManyThrough(entity);
 
             await _context.SaveChangesAsync();
 

src/JsonApiDotNetCore/Models/HasManyThroughAttribute.cs

@@ -95,6 +95,21 @@ public HasManyThroughAttribute(string publicName, string internalThroughName, Li
         /// </example>
         public PropertyInfo LeftProperty { get; internal set; }
 
+        /// <summary>
+        /// The identity property back to the parent resource.
+        /// </summary>
+        /// 
+        /// <example>
+        /// In the `[HasManyThrough("tags", nameof(ArticleTags))]` example
+        /// this would point to the `Article.ArticleTags.ArticleId` property
+        ///
+        /// <code>
+        /// public int ArticleId { get; set; }
+        /// </code>
+        ///
+        /// </example>
+        public PropertyInfo LeftIdProperty { get; internal set; }
+
         /// <summary>
         /// The navigation property to the related resource from the join type.
         /// </summary>
@@ -110,6 +125,32 @@ public HasManyThroughAttribute(string publicName, string internalThroughName, Li
         /// </example>
         public PropertyInfo RightProperty { get; internal set; }
 
+        /// <summary>
+        /// The identity property back to the related resource from the join type.
+        /// </summary>
+        /// 
+        /// <example>
+        /// In the `[HasManyThrough("tags", nameof(ArticleTags))]` example
+        /// this would point to the `Article.ArticleTags.TagId` property
+        ///
+        /// <code>
+        /// public int TagId { get; set; }
+        /// </code>
+        ///
+        /// </example>
+        public PropertyInfo RightIdProperty { get; internal set; }
+
+        /// <summary>
+        /// Get the name of the identity property from the navigation property.
+        /// </summary>
+        /// 
+        /// <example>
+        /// In the `[HasManyThrough("tags", nameof(ArticleTags))]` example
+        /// this would translate to the `Article.ArticleTags.Tag` property
+        /// to TagId. This is then used to set the Left and RightIdProperties.
+        /// </example>
+        public virtual string GetIdentityPropertyName(PropertyInfo property) => property.Name + "Id";
+
         /// <summary>
         /// The join entity property on the parent resource.
         /// </summary>