Sum in child collection - mysql

Question

Using MySQL with EF6 throws an exception when I sum values from an empty child collection as the DefaultIfEmpty is not supported well with MySQL as related in bug #80127.
public class Foo
{
public int Id { get; set; }
public decimal Total { get; set; }
public virtual IList<Bar> Bars { get; set; }
}
public class Bar
{
public int Id { get; set; }
public int FooId { get; set; }
public virtual Foo Foo { get; set; }
public decimal Received { get; set; }
}
Using the recommended approach with DefaultIfEmpty throws an exception with invalid where clausule 'Project1'.'Id'. This is an old bug of MySQL.
var result = db.Foo.Select(f => new {
Total = f.Total,
Received = f.Bars.Select(b => b.Received).DefaultIfEmpty().Sum()
});
I'm using an inline if that works fine but generates an very ugly SQL with lots of inner queries and repetitions of select statements.
var result = db.Foo.Select(f => new {
Total = f.Total,
Received = f.Bars.Any() ? f.Bars.Sum(b => b.Received) : 0
});
Is there a better way to avoid DefaultIfEmpty?

Answer 1

The alternative of DefaultIfEmpty which I usually prefer is using cast operator to promote the non nullable type to nullable, which works (even) with MySQL connector.
Then the solution depends of your receiver class property type.
The best is if you can receive a nullable result, in which case the query is simple:
var result = db.Foo.Select(f => new {
Total = f.Total,
Received = f.Bars.Sum(b => (decimal?)b.Received)
});
If it needs to be a non nullable type, you can use null coalescing operator
var result = db.Foo.Select(f => new {
Total = f.Total,
Received = f.Bars.Sum(b => (decimal?)b.Received) ?? 0
});
but the generated SQL query is ugly and inefficient.
The best you can do in such case is to use (a quite annoying) double select trick:
var result = db.Foo.Select(f => new {
f.Total,
Received = f.Bars.Sum(b => (decimal?)b.Received)
})
.Select(r => new {
r.Total,
Received = r.Received ?? 0
};
or (a quite better) query syntax with let clause:
var result =
from f in db.Foos
let received = f.Bars.Sum(b => (decimal?)b.Received)
select new { f.Total, Received = received ?? 0 };
Tested on latest EF6.1.3 with MySQL Connector/Net 6.9.8