LINQ+EF：Cannot use multiple context instances within a single query execution. Ensure the query uses a single context instance.

//1.查询账套数据库,获取销售单数据
var q = from a in _Database.GetQueryable<tb_SO>()
        where a.FlagApp == "Y" && a.DocDate >= DateTime.Today.AddMonths(-1)//统计已审核,1个月订单
        group a by a.CreatedBy into g
        select new HomeDataSalesAmount
        {
            UserId = g.Key,
            Rows = g.Count(),
            Amount = g.Sum(s => s.TotalAmount ?? 0),
        };

var listSalesRank = q.Where(w => w.Amount > 0).OrderByDescending(o => o.Amount).ToList();
listSalesRank.ForEach(item => item.Rank = listSalesRank.IndexOf(item) + 1);//排名

//2.查询系统数据库,获取用户表
var listUsers = DatabaseProvider.SystemDatabase.GetQueryable<tb_MyUser>()
    .Select(s => new { s.Account, s.UserName }).ToList();

//3.关联起来，更新姓名
(from a in listSalesRank
 join b in listUsers on a.UserId equals b.Account
 select new { a, b }).ForEach(item =>
 {
     item.a.UserName = item.b.UserName;
 });