How to write subquery in select list in EF Core? - entity-framework-core

Question

Select *,
(Select DefaultStartDay from Scheduler.ProgramSettings ps where ps.DefaultStartDay = s.Id ) [DefaultStartDay]
from Scheduler.Schedules s
where ScheduleType = 2;
I want to write above SQL query in EF Core, Specially I need subquery in select list to get data from another table with specific condition.
please refer image.Sample Data with SQL Query
I have tried below EF Core but getting wrong result.
var model = _context.Schedules
.Where(s => s.ScheduleType == 2)
.Select(rv => new ProgramSetting
{
Id = rv.Id,
ProgramTemplateId = rv.ProgramTemplateId,
IsActive = rv.IsActive,
DefaultStartDay = rv.Id
}).ToArray();

Answer 1

The SQL query is wrong and this is a misuse of EF Core.
First, that SQL will fail if there's more than 1 result from the subquery. Even in SQL you'd need a different query. An INNER JOIN would return the same results without failing if there are multiple matches.
Select s.*,ps.DefaultStartDay
from Scheduler.Schedules s
inner join Scheduler.ProgramSettings ps on ps.DefaultStartDay = s.Id
where ScheduleType = 2;
Second, using LINQ to emulate SQL is a misuse of both EF Core and LINQ. EF isn't a replacement for SQL, it's an ORM. Its job is to give the impression of working with in-memory objects instead of tables, not allow you to write SQL queries in C#
It's the ORM's job to generate JOINs as needed from the relations between entities (not tables). In this case, if Schedule has a ProgramSettins property, EF would generate the necessary joins automatically. Loading an entire schedule object could be as simple as :
var schedules=_context.Schedules
.Incule(sch=>sch.ProgramSettings)
.Where(s => s.ScheduleType == 2)
.ToArray();
Include is used to eagerly load the settings, not to force a JOIN.
If a Select clause is used that requires a property from ProgramSettings, the JOIN will be generated automatically, eg :
var namesAndDays=_context.Schedules
.Where(s => s.ScheduleType == 2)
.Select(s=>new {
Name = s.Name,
StartDay = s.ProgramSettings.DefaultStartDay
})
.ToArray();