Question

My database has two tables, Reads and Alarms. There is a one to many relationship from the Reads (1) to the Alarms (many) table. For various reasons, the foreign key constraint on the Alarms table was deleted. I cannot restore this constraint. The Read object in my Entity Framework model consequently does not have an Alarms navigation property.

I have a boolean property in my ReadViewModel class called HasAlarms. I want this to be set to true if there is at least one row in the Alarms table that is related to the Read. I know that if the Alarms navigation property were there, I could do this with something like this:

var reads = from read in context.Reads
            select new ReadViewModel { . . . };

foreach ( ReadViewModel read in reads ) {
    read.HasAlarms = read.Alarms.Any();
}

However, I don’t have the Alarms navigation property any more. What is the most efficient way to do this in my current situation?

Tony

Answer 1

You’ll need something like this:

foreach ( ReadViewModel read in reads ) {
    read.HasAlarms = context.Alarms.Any(a => a.ReadId == read.Id);
}

Also you can perform join, to get state in one query:

var results = from r in reads
join a in context.Alarms on r.Id equals a.ReadId into ra // perform a left join
from a in ra.DefaultIfEmpty()                            // on Alarms
select new { Read = r, HasAlarm = a != null };

foreach (var res in results)
   res.Read.HasAlarm = res.HasAlarm;