IQueryable Parent-Child - linq-to-sql

Question

I need to cast: IQueryable<PARENT> to IQueryable<Child>.
IQueryable<PARENT> query = GetParents();
IQueryable<CHILD> result = query.Select(t => t.Children);
This doesn't work, unable to convert EntitySet to IQueryable. Thanks

Answer 1

Use:
IQueryable<CHILD> result = query.SelectMany(t => t.Children);
The query returns all cild items of the parent, childs referenced by multiple parents will be returned multiple times.
To get the distinct rows use - you will have to implement a class that Implements IComparer though, to do a custom filtering. Distinct() will look at the instances and not the values.
IQueryable<CHILD> result = query.SelectMany(t => t.Children)
.Distict(new Comparer<CHILD>());
Distinct() removes duplicats from the query.
Aternatively you can use grouping to create a grouping for the children:
IEnumerable<CHILD> result = query.SelectMany(t => t.Children)
.GroupBy(x => new CHILD {
id = x.id
// add other properties here
})
.Select(g => g.Key);
Instead of new CHILD { } you could also use new { } to create an anonymous result, in this case you should replace IEnumerable<CHILD> with var.
To perform a type cast in Linq you can use - although I would assume that this will not work in you case:
someQuery.OfType<TypeToCastTo>().Cast<TypeToCastTo>();
OfType<T>() filters the items so that Cast<T>() can perform the type cast.