Get nested data and sharp into DTO with nested DTO - entity-framework

Question

I'm newbie to EF, Linq and C# in general, I'm stuck with developing following.
I cannot map data into structure like this:
Id,
Actions [
Action1,
Action2,
Action3
]
I have 2 DTO classes like this:
public class TestDTO
{
public int TestId { get; set; }
public TestDTO2[] Actions { get; set; }
}
and
public class TestDTO2
{
public int TestActionId { get; set; }
public DateTime? StartDate { get; set; }
...
}
I've separated calls to DB into file called BusinessLogic, I'm doing it like this:
public IQueryable<TestDTO> GetNested(Filter filter)
{
var query =
from a in db.Table1.AsQueryable()
select new TestDTO
{
TestId = a.Id,
Actions = (
from b in db.Table2.AsQueryable()
where a.Id == b.TestId
select new TestDTO2
{
TestActionId = b.TestActionId,
StartDate = b.StartDate
}
).ToArray()
};
return query;
}
I'm getting following error:
LINQ to Entities does not recognize the method 'Project.Core.Models.TestDTO2[] ToArrayTestDTO2' method, and this method cannot be translated into a store expression.

Answer 1

You can't perform exactly this query, it is better to make two simple queries and then process their results on client side:
var main = db.Table1.Select(x => new { x.Id, x.Title }).ToList();
var mainIds = main.Select(x => x.Id).ToList();
var actions = db.Table2.Where(x => mainIds.Contains(x.TestId)).Select(x => new
{
x.TestId,
x.TestActionId,
x.StartDate
}).ToList();
var result = main.Select(x => {
var actions = actions.Where(y => y.TestId == x.Id).Select(y => new TestDTO2
{
TestActionId = y.TestActionId,
StartDate = y.StartDate
}).ToArray();
return new TestDTO
{
TestId = x.Id,
Title = x.Title,
Actions = actions.Length == 0 ? null : actions
};
}).ToList();

Answer 2

yes, you can't use any c# method that can't translate a sql in EF.
actually, you need get a list,then covert it to your DTO
db.Table1
.Join(db.Table2,
a => a.Id,
b => b.TestId,
(a, b) => new
{
a.Id,
b
})
.GroupBy(k => k.Id, v => v).ToList()
.Select(a=>new TestDTO
{
TestId = a.Id,
Actions = a.Select(b=>
new TestDTO2
{
TestActionId = b.TestActionId,
StartDate = b.StartDate
}.ToArray()
}).ToList()