返回指定字段:
//返回一个字段 List<int> t5 = fsql.Select<Topic>().ToList(a => a.Id); //返回匿名类 List<匿名类> t6 = fsql.Select<Topic>().ToList(a => new { a.Id, a.Title }); //返回元组 List<(int, string)> t7 = fsql.Select<Topic>().ToList<(int, string)>("id, title"); //返回SQL字段 List<匿名类> t8 = fsql.Select<Topic>().ToList(a => new { a.Id, a.Title, a.Type, //可以直接返回导航属性 Type cstitle = "substr(a.title, 0, 2)", //将 substr(a.title, 0, 2) 作为查询字段 csnow = Convert.ToDateTime("now()"), //将 now() 作为查询字段 //奇思妙想:怎么查询开窗函数的结果 }); //返回子查询的字段 List<匿名类> t9 = fsql.Select<Topic>().ToList(a => new { a.Id, count = fsql.Select<T2>().Count(), max = fsql.Select<T2>().Max(b => b.Id), min = fsql.Select<T2>().Min(b => b.Id), name = fsql.Select<2>().First(b => b.name) });
关于忽略某些字段的功能,因为多表机制不好确定,所以没提供官方接口,不过可以通过以下扩展方法来实现:
public static List<T1> ToListIgnore<T1>(this ISelect<T1> that, Expression<Func<T1, object>> selector) { if (selector == null) return that.ToList(); var s0p = that as Select0Provider; var tb = s0p._tables[0]; var parmExp = tb.Parameter ?? Expression.Parameter(tb.Table.Type, tb.Alias); var initExps = tb.Table.Columns.Values .Where(a => a.Attribute.IsIgnore == false) .Select(a => new { exp = Expression.Bind(tb.Table.Properties[a.CsName], Expression.MakeMemberAccess(parmExp, tb.Table.Properties[a.CsName])), ignored = TestMemberExpressionVisitor.IsExists(selector, Expression.MakeMemberAccess(parmExp, tb.Table.Properties[a.CsName])) }) .Where(a => a.ignored == false) .Select(a => a.exp) .ToArray(); var lambda = Expression.Lambda<Func<T1, T1>>( Expression.MemberInit( Expression.New(tb.Table.Type), initExps ), parmExp ); return that.ToList(lambda); } class TestMemberExpressionVisitor : ExpressionVisitor { public string MemberExpString; public bool Result { get; private set; } public static bool IsExists(Expression selector, Expression memberExp) { var visitor = new TestMemberExpressionVisitor { MemberExpString = memberExp.ToString() }; visitor.Visit(selector); return visitor.Result; } protected override Expression VisitMember(MemberExpression node) { if (!Result && node.ToString() == MemberExpString) Result = true; return node; } }
参考网址: https://www.cnblogs.com/FreeSql/p/11531376.html