Update of navigation property in EF7 does not get saved

I have simple model:

public class Post
{
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public long Id { set; get; }
        //non-important properties stripped, to focus on problem
        public virtual Resource Resource { set; get; }

        public virtual ICollection<Tag> Tags { set; get; }
}
public class Resource
{
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public Guid Id { set; get; }

        [Url]
        public string Url { set; get; }
}

and DbContext (I use ASP.NET identity in this project, if this is relevant):

public class DbContext : IdentityDbContext<User>
{
       protected override void OnModelCreating(ModelBuilder modelBuilder)
       {
            base.OnModelCreating(modelBuilder);
            var postEntity = modelBuilder.Entity<Post>();
            postEntity.Reference(p => p.Resource).InverseCollection(); //no navigation property on Resource side
            postEntity.Collection(p => p.Tags).InverseReference(tag => tag.Post);
            postEntity.ToTable("Posts");

            var resourceEntity = modelBuilder.Entity<Resource>();
            resourceEntity.ToTable("Resources");

            var tagEntity = modelBuilder.Entity<Tag>();
            tagEntity.Reference(t => t.Post).InverseCollection(p => p.Tags).Required(false);
            tagEntity.ToTable("Tags");
       }
}

After migration (SQL Server), database tables looks like expected - Post table has Foreign Key to ResourceId.

Creating Post's works fine, when I attach post.Resource (already created Resource).

My problem occurs when I want to replace post.Resource. By replace, I mean selecting one of already existing Resources and assigning it to post.

var resource2 = Database.Resources.First(r=>r.Url == "xyz");

I have tried:

1. post.Resource = resource2; Database.Entry(post).State = EntityState.Modified;

2. Database.Entry(post).Property(p => p.Resource).CurrentValue = resource2;

3. post.Resource = null;

With different combinations of them also, but none of them works. After calling await SaveChangesAsync(); and looking up in database - there are no changes. How to perform the replace (update of foreign key) properly?

//Update 14.09.2015 Issue was caused by additional select, performed to update One-To-Many relationship. Full code:

        var Database new DbContext();
        var resource2 = Database.Resources.First(r=>r.Url == "xyz");
        var oldAssignedTags = Database.Posts.Include(p=>p.Tags).FirstOrDefault(p => p.Id == post.Id).Tags;
        var tags = newTags as List<Tag> ?? newTags.ToList();
        //TagComparer is irrelevant here
        var toRemove = oldAssignedTags.Except(tags, TagComparer);
        var toAdd = tags.Except(oldAssignedTags, TagComparer);

        foreach (var tag in toRemove)
        {
            Database.Entry(tag).State = EntityState.Deleted; //Database.Tags.Remove(tag);
        }

        foreach (var tag in toAdd)
        {
            tag.Post = post;
            post.Tags.Add(tag);
        }

        post.Resource = resource2;
        await Database.SaveChangesAsync();