c#通过表达式树优雅的实现分组取TopN笔记

需要引入nuget包来实现ef.functions调用row_number

Thinktecture.EntityFrameworkCore.SqlServer

调用方式：

//顺排
context.Table.GroupBySortTop(1, x => x.partitionProptery, x => x.orderByProperty).ToList()
//倒排
context.Table.GroupBySortTopDescending(1, x => x.partitionProptery, x => x.orderByProperty).ToList()

核心代码

    public class GroupBySorting<T>
    {
        public T Data { get; set; }
        public long Rowid { get; set; }
    }
    public static class Extension
    {
        static string OrderBy = nameof(RelationalDbFunctionsExtensions.OrderBy);
        static string OrderByDescending = nameof(RelationalDbFunctionsExtensions.OrderByDescending);
        static MethodInfo RowNumber = typeof(RelationalDbFunctionsExtensions).GetMethods().FirstOrDefault(x => x.IsGenericMethod && x.Name == nameof(RelationalDbFunctionsExtensions.RowNumber));
        static Dictionary<string, MethodInfo> OrderMethods = new List<(string, MethodInfo)>() { 
            new (OrderBy, typeof(RelationalDbFunctionsExtensions).GetMethod(OrderBy)),
            new (OrderByDescending, typeof(RelationalDbFunctionsExtensions).GetMethod(OrderByDescending)) }.ToDictionary(x => x.Item1, x => x.Item2);
        static IQueryable<T> GroupBySortTop<T, T1, T2>(this IQueryable<T> source, int limit, Expression<Func<T, T1>> partitionByExpression, Expression<Func<T, T2>> orderByExpression, string orderBy)
        {
            var prm = Expression.Parameter(typeof(T));
            var constFunc = Expression.Constant(EF.Functions);
            var orderFunc = Expression.Call(null, OrderMethods[orderBy].MakeGenericMethod(typeof(int)), constFunc, Expression.Property(prm, (orderByExpression.Body as MemberExpression).Member.Name));
            var expressions = Expression.Call(null, RowNumber.MakeGenericMethod(typeof(T1)), constFunc, Expression.Property(prm, (partitionByExpression.Body as MemberExpression).Member.Name), orderFunc);
            var targetType = typeof(GroupBySorting<T>);
            var memberBindings = new List<MemberBinding>();
            memberBindings.Add(Expression.Bind(targetType.GetProperty(nameof(GroupBySorting<T>.Data)), prm));
            memberBindings.Add(Expression.Bind(targetType.GetProperty(nameof(GroupBySorting<T>.Rowid)), expressions));
            var initmember = Expression.MemberInit(Expression.New(typeof(GroupBySorting<T>)), memberBindings);
            var lambda = Expression.Lambda<Func<T, GroupBySorting<T>>>(initmember, prm);
            return source.Select(lambda).AsSubQuery().Where(x => x.Rowid <= limit).Select(x => x.Data);
        }
        public static IQueryable<T> GroupBySortTop<T, T1, T2>(this IQueryable<T> source, int limit, Expression<Func<T, T1>> partitionByExpression, Expression<Func<T, T2>> orderByExpression)
        {
            return GroupBySortTop(source, limit, partitionByExpression, orderByExpression, OrderBy);
        }
        public static IQueryable<T> GroupBySortTopDescending<T, T1, T2>(this IQueryable<T> source, int limit, Expression<Func<T, T1>> partitionByExpression, Expression<Func<T, T2>> orderByExpression)
        {
            return GroupBySortTop(source, limit, partitionByExpression, orderByExpression, OrderByDescending);
        }
    }