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。

巴特西

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

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