port @jasolko 's changes

Author: jaredcnance

.gitignore

@@ -1,3 +1,5 @@
+.DS_Store
+
 ## Ignore Visual Studio temporary files, build results, and
 ## files generated by popular Visual Studio add-ons.
 

src/Examples/JsonApiDotNetCoreExample/Data/AppDbContext.cs

@@ -53,7 +53,8 @@ protected override void OnModelCreating(ModelBuilder modelBuilder)
         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; }
     }
 }

src/Examples/JsonApiDotNetCoreExample/Models/Article.cs

@@ -1,3 +1,5 @@
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations.Schema;
 using JsonApiDotNetCore.Models;
 
 namespace JsonApiDotNetCoreExample.Models
@@ -10,5 +12,10 @@ public class Article : Identifiable
         [HasOne("author")]
         public Author Author { get; set; }
         public int AuthorId { get; set; }
+
+        [NotMapped]
+        [HasManyThrough("tags", nameof(ArticleTags))]
+        public List<Tag> Tags { get; set; }
+        public List<ArticleTag> ArticleTags { get; set; }
     }
 }

src/Examples/JsonApiDotNetCoreExample/Models/ArticleTag.cs

@@ -0,0 +1,13 @@
+using JsonApiDotNetCore.Models;
+
+namespace JsonApiDotNetCoreExample.Models
+{
+    public class ArticleTag : Identifiable
+    {
+        public int ArticleId { get; set; }
+        public Article Article { get; set; }
+
+        public int TagId { get; set; }
+        public Tag Tag { get; set; }
+    }
+}

src/Examples/JsonApiDotNetCoreExample/Models/Tag.cs

@@ -0,0 +1,9 @@
+using JsonApiDotNetCore.Models;
+
+namespace JsonApiDotNetCoreExample.Models
+{
+    public class Tag : Identifiable
+    {
+        public string Name { get; set; }
+    }
+}

src/JsonApiDotNetCore/Builders/ContextGraphBuilder.cs

@@ -166,7 +166,25 @@ protected virtual List<RelationshipAttribute> GetRelationships(Type entityType)
                 attribute.InternalRelationshipName = prop.Name;
                 attribute.Type = GetRelationshipType(attribute, prop);
                 attributes.Add(attribute);
+
+                if(attribute is HasManyThroughAttribute hasManyThroughAttribute) {
+                    var throughProperty = properties.SingleOrDefault(p => p.Name == hasManyThroughAttribute.InternalThroughName);
+                    if(throughProperty == null)
+                        throw new JsonApiSetupException($"Invalid '{nameof(HasManyThroughAttribute)}' on type '{entityType}'. Type does not contain a property named '{hasManyThroughAttribute.InternalThroughName}'.");
+                    
+                    // assumption: the property should be a generic collection, e.g. List<ArticleTag>
+                    if(prop.PropertyType.IsGenericType == false)
+                        throw new JsonApiSetupException($"Invalid '{nameof(HasManyThroughAttribute)}' on type '{entityType}'. Expected through entity to be a generic type, such as List<{prop.PropertyType}>.");
+                    hasManyThroughAttribute.ThroughType = prop.PropertyType.GetGenericArguments()[0];
+
+                    var throughProperties = hasManyThroughAttribute.ThroughType.GetProperties();
+                    // Article → ArticleTag.Article
+                    hasManyThroughAttribute.LeftProperty = throughProperties.Single(x => x.PropertyType == entityType);
+                    // Article → ArticleTag.Tag
+                    hasManyThroughAttribute.RightProperty = throughProperties.Single(x => x.PropertyType == hasManyThroughAttribute.Type);
+                }
             }
+
             return attributes;
         }
 

src/JsonApiDotNetCore/Extensions/DbContextExtensions.cs

@@ -11,6 +11,16 @@ public static class DbContextExtensions
         public static DbSet<T> GetDbSet<T>(this DbContext context) where T : class 
             => context.Set<T>();
 
+        /// <summary>
+        /// Get the DbSet when the model type is unknown until runtime
+        /// </summary>
+        public static IQueryable<object> Set(this DbContext context, Type t)
+            => (IQueryable<object>)context
+                .GetType()
+                .GetMethod("Set")
+                .MakeGenericMethod(t) // TODO: will caching help runtime performance?
+                .Invoke(context, null);
+
         /// <summary>
         /// Determines whether or not EF is already tracking an entity of the same Type and Id
         /// </summary>

src/JsonApiDotNetCore/Internal/Generics/GenericProcessor.cs

@@ -1,7 +1,10 @@
+using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Linq.Expressions;
 using System.Threading.Tasks;
 using JsonApiDotNetCore.Data;
+using JsonApiDotNetCore.Extensions;
 using JsonApiDotNetCore.Models;
 using Microsoft.EntityFrameworkCore;
 
@@ -21,16 +24,41 @@ public GenericProcessor(IDbContextResolver contextResolver)
             _context = contextResolver.GetContext();
         }
 
-        public async Task UpdateRelationshipsAsync(object parent, RelationshipAttribute relationship, IEnumerable<string> relationshipIds)
+        public virtual async Task UpdateRelationshipsAsync(object parent, RelationshipAttribute relationship, IEnumerable<string> relationshipIds)
         {
             SetRelationships(parent, relationship, relationshipIds);
 
             await _context.SaveChangesAsync();
         }
 
-        public void SetRelationships(object parent, RelationshipAttribute relationship, IEnumerable<string> relationshipIds)
+        public virtual void SetRelationships(object parent, RelationshipAttribute relationship, IEnumerable<string> relationshipIds)
         {
-            if (relationship.IsHasMany)
+            if (relationship is HasManyThroughAttribute hasManyThrough)
+            {
+                var parentId = ((IIdentifiable)parent).StringId;
+                ParameterExpression parameter = Expression.Parameter(hasManyThrough.Type);
+                Expression property = Expression.Property(parameter, hasManyThrough.LeftProperty);
+                Expression target = Expression.Constant(parentId);
+                Expression toString = Expression.Call(property, "ToString", null, null);
+                Expression equals = Expression.Call(toString, "Equals", null, target);
+                Expression<Func<object, bool>> lambda = Expression.Lambda<Func<object, bool>>(equals, parameter);
+
+                var oldLinks = _context
+                    .Set(hasManyThrough.ThroughType)
+                    .Where(lambda.Compile())
+                    .ToList();
+
+                _context.Remove(oldLinks);
+
+                var newLinks = relationshipIds.Select(x => {
+                    var link = Activator.CreateInstance(hasManyThrough.ThroughType);
+                    hasManyThrough.LeftProperty.SetValue(link, TypeHelper.ConvertType(parent, hasManyThrough.LeftProperty.PropertyType));
+                    hasManyThrough.RightProperty.SetValue(link, TypeHelper.ConvertType(x, hasManyThrough.RightProperty.PropertyType));
+                    return link;
+                });
+                _context.AddRange(newLinks);
+            }
+            else if (relationship.IsHasMany)
             {
                 var entities = _context.Set<T>().Where(x => relationshipIds.Contains(x.StringId)).ToList();
                 relationship.SetValue(parent, entities);

src/JsonApiDotNetCore/Models/HasManyThroughAttribute.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Reflection;
+
+namespace JsonApiDotNetCore.Models
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    /// 
+    /// <example>
+    /// 
+    /// </example>
+    public class HasManyThroughAttribute : HasManyAttribute
+    {
+        /// <summary>
+        /// 
+        /// </summary>
+        /// <example>
+        /// 
+        /// </example>
+        public HasManyThroughAttribute(string publicName, string internalThroughName, Link documentLinks = Link.All, bool canInclude = true)
+        : base(publicName, documentLinks, canInclude)
+        {
+            InternalThroughName = internalThroughName;
+        }
+
+        public string InternalThroughName { get; private set; }
+        public Type ThroughType { get; internal set; }
+
+        /// <summary>
+        /// The navigation property back to the parent resource.
+        /// </summary>
+        /// 
+        /// <example>
+        /// In the `[HasManyThrough("tags", nameof(ArticleTags))]` example
+        /// this would point to the `Article.ArticleTags.Article` property
+        ///
+        /// <code>
+        /// public Article Article { get; set; }
+        /// </code>
+        ///
+        /// </example>
+        public PropertyInfo LeftProperty { get; internal set; }
+
+        /// <summary>
+        /// The navigation property to the related resource.
+        /// </summary>
+        /// 
+        /// <example>
+        /// In the `[HasManyThrough("tags", nameof(ArticleTags))]` example
+        /// this would point to the `Article.ArticleTags.Tag` property
+        ///
+        /// <code>
+        /// public Tag Tag { get; set; }
+        /// </code>
+        ///
+        /// </example>
+        public PropertyInfo RightProperty { get; internal set; }
+    }
+}