比较匹配元素在Iqueryable中使用单个查询

Question

我想比较两个不同表中的两个列表UserGroup使用单个查询，即通过不多次访问数据库。

当前我在一个查询中提取所有分配的UserGroup，并与其他查询中允许的所有Usergroup进行比较。

var query = _context.AppSubOperationUserGroupMappings.Where(filterPredicate) 
                  .Select(x => x.UserGroup) 
                  .ToList(); 

    var allowedUserGroups = _context.Users.Where(x => x.Id == userId) 
               .Select(x => x.UserGroupUserMappings.Select(y => y.UserGroup)) 
               .First() 
               .ToList(); 

    return query.Any(a => allowedUserGroups.Any(b => b.Id == a.Id)); 

如何将它们合并到单个查询中？

Answer 1

删除ToList和首先，使用加入和的SelectMany

var query = _context.AppSubOperationUserGroupMappings.Where(filterPredicate) 
    .Select(x => x.UserGroup); 

var allowedUserGroups = _context.Users.Where(x => x.Id == userId) 
    .SelectMany(x => x.UserGroupUserMappings, (x, y) => y.UserGroup); 

return query 
    .Join(
     allowedUserGroups, 
     x => x.Id, 
     x => x.Id, 
     (x, y) => false) // doesn't matter what to select 
    .Any();