using EfCoreOverview_Demo.Entities;
using System;
using System.Linq;
using System.Linq.Expressions;

namespace EfCoreOverview_Demo
{
    class Program
    {
        static void Main(string[] args)
        {
            var context = new TelerikAcademyContext();
            var minSalaryGroups = context.Employees
                .GroupBy(e => e.DepartmentId, (key, group) => new
                {
                    DepartmentId = key,
                    MinSalary = group.Min(e => e.Salary)
                })
                .ToList();

            var predicate = minSalaryGroups.Aggregate(
                ToExpression((Employees x) => false),
                (expr, next) => expr.Or(e => e.DepartmentId == next.DepartmentId &&
                                             e.Salary == next.MinSalary)
            );

            var minSalaryEmployees = context.Employees
                .Where(predicate)
                .Select(e => new
                {
                    e.FirstName,
                    e.LastName,
                    e.Salary,
                })
                .ToList();

        }

        public static Expression<Func<T, R>> ToExpression<T, R>(Expression<Func<T, R>> e) => e;
    }

    public static class ExpressionExtensions
    {
        public static Expression<Func<T, bool>> Or<T>(
            this Expression<Func<T, bool>> left,
            Expression<Func<T, bool>> right)
                => Expression.Lambda<Func<T, bool>>(
                       Expression.OrElse(
                           left.Body,
                           Expression.Invoke(right,left.Parameters.Cast<Expression>())),
                       left.Parameters);

    }
}