linq to sql: 在Entityfamework Core中使用多个DbContext

最近在学习DotNetCore并做一个自己的小项目，分为了多个数据库，AccountDbContext和BlogDbContext,
发blog的时候需要用到Account的信息，但是再Blog中只记录了UserID，然后用到了如下代码：

var q = from u in this.DbAccount.Users
        join p in this.DbBlog.Posts
        on u.UserID equals p.UserID
        select u;
var v = q.FirstOrDefault();

但是目前来说貌似不支持，通过各种搜索，倒是有一个改xml的方案，但是我的项目的DbContext是手写而不是使用SqlMetal工具生成的。报错部分信息如下

"Microsoft.EntityFrame
workCore.Query","queryModel":"(from User u in DbSet<User> join Post p in DbSet<Post> on [u].UserID equals [p].UserID select [u]).F..."}}}}
'q.FirstOrDefault()' threw an exception of type 'System.ArgumentNullException'
    Data: {System.Collections.ListDictionaryInternal}
    HResult: -2147467261
    HelpLink: null
    InnerException: null
    Message: "Value cannot be null.\r\nParameter name: entityType"
    ParamName: "entityType"
    Source: "Microsoft.EntityFrameworkCore.Relational"
    StackTrace: "   at Microsoft.EntityFrameworkCore.Utilities.Check.NotNull[T](T value, String parameterName)\r\n   at Microsoft.EntityFrameworkCore.RelationalMetadataExtensions.Relational(IEntityType entityType)\r\n   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.RelationalEntityQueryableExpressionVisitor.VisitEntityQueryable(Type elementType)

但是在同一个server上夸数据库的查询再sqlserver和mysql都是支持的。也就是说EFCore 2.0现在不支持，现在临时的解决方案如下，

            var userID = this.User.GetUserID();

            var queryUsers = from u in this.DbAccount.Users.Include(a => a.Account)
                             where u.UserID == userID
                             select new
                             {
                                 UserID = u.UserID,
                                 UserNickName = u.NickName
                             };
            var queryPosts = from p in this.DbBlog.Posts.Include(a => a.Tags)
                             join pc in this.DbBlog.PostContents
                             on p.PostID equals pc.PostID
                             orderby pc.CreateAt descending
                             where p.UserID == userID
                             select new
                             {
                                 PostID = p.PostID,
                                 UserID = p.UserID,
                                 Title = p.Title,
                                 URL = p.URL,
                                 Summary = p.Summary,
                                 Tags = p.Tags,
                                 EditorType = pc.EditorType,
                                 MD5Hash = pc.MD5Hash,
                                 Content = pc.Content,
                                 ContentCreateAt = pc.CreateAt
                             };

            var queryPo = from u in queryUsers.ToList()
                          join p in queryPosts.ToList()
                          on u.UserID equals p.UserID
                          orderby p.ContentCreateAt
                          select new
                          {
                              UserNickName = u.UserNickName,
                              Title = p.Title
                          };

            var pa = queryPo.ToList();

测试通过，但是要麻烦多了，就看EFCore什么时候能原生支持了。