Question

我对C＃编程很陌生。我有一种情况需要从数据库表中动态读取数据，我需要根据类别切换表。我从表中读取的数据总是相同的，只是表格不同。是否可以在查询中动态切换数据库表？

我做了一个抽象的代码来解释我的想法：

                    var userPost = if(Category == "News"){ db.News } else { db.Sport}
                    .Include(x => x.Popular)
                    .Include(x => x.Popular.Category)
                    .Where(x => x.User.Id == userId)
                    .OrderByDescending(x => x.CreatedAt)
                    .ProjectTo<PostViewModel>(_mapper.ConfigurationProvider)
                    .ToPagedList(pageIndex, pageSize);

谢谢;）

Answer 1

假设您的ORM将两个表映射到相同的类型，您应该能够这样做：

var userPost = (Category == "News" ? db.News : db.Sport)
                .Include(x => x.Popular)
                .Include(x => x.Popular.Category)
                .Where(x => x.User.Id == userId)
                .OrderByDescending(x => x.CreatedAt)
                .ProjectTo<PostViewModel>(_mapper.ConfigurationProvider)
                .ToPagedList(pageIndex, pageSize);

或者，或许更具可读性：

var table = (Category == "News" ? db.News : db.Sport);

var userPost = table
                .Include(x => x.Popular)
                .Include(x => x.Popular.Category)
                .Where(x => x.User.Id == userId)
                .OrderByDescending(x => x.CreatedAt)
                .ProjectTo<PostViewModel>(_mapper.ConfigurationProvider)
                .ToPagedList(pageIndex, pageSize);

如果要复用多个表，则可能需要将选择逻辑拆分为另一种方法：

internal IQueryable<ROWTYPE> SelectTable(string category)
{
    switch (category)
    {
        case "News": return db.News;
        case "Sport": return db.Sport;
        case "Arts": return db.Arts;
        default: throw new ArgumentException("Unsupported category: " + category);
    }
} 



var table = SelectTable(Category);

var userPost = table
                .Include(x => x.Popular)
                .Include(x => x.Popular.Category)
                .Where(x => x.User.Id == userId)
                .OrderByDescending(x => x.CreatedAt)
                .ProjectTo<PostViewModel>(_mapper.ConfigurationProvider)
                .ToPagedList(pageIndex, pageSize);

是否可以在查询中动态切换数据库表？

1 个答案: