Nested Tables Using a DTO - entity-framework

Question

I need help getting my WebApi Controller to work.
I have a 3 table Models like this.
First Table
public class MainTable{
public int MainTableID { get; set; }
... Other Fields
public ICollection<SubTable> SubTables { get; set; }
}
Second Table
public class SubTable{
public int SubTableID { get; set; }
... Other Fields
public int MainTableID { get; set; }
[ForeignKey("MainTableID ")]
[JsonIgnore]
public virtual MainTable MainTable{ get; set; }
public ICollection<SubSubTable> SubSubTables { get; set; }
}
Third Table
public class SubSubTable{
public int SubSubTableID { get; set; }
... Other Fields
public int SubTableID { get; set; }
[ForeignKey("SubTableID")]
[JsonIgnore]
public virtual SubTable SubTable{ get; set; }
}
I need to flatten the first model because of other relationships not mentioned in this post so I am using a dto
DTO
public class TableDTO
{
public int MainTableID { get; set; }
... Other Fields (there is a lot of flattening happening here but I am going to skip it to keep this simple)
public ICollection<SubTable> SubTables { get; set; }
}
Now that I got all of that out of the way. To my question.. I am linking this all to a web api controller.
If I use the DTO and create a controller like this
Controller with DTO
public IQueryable<TableDTO> GetMainTable()
{
var mainTable = from b in db.MainTables
.Include(b => b.SubTable.Select(e => e.SubSubTable))
select new TableDTO()
{
MainTableID = b.MainTableID
eager mapping of all the fields,
SubTables = b.SubTables
};
return mainTable;
}
This works for everything except the SubSubTable which returns null. If I ditch the DTO and create a controller like this
Controller without DTO
public IQueryable<MainTable> GetMainTable()
{
return db.MainTables
.Include(c => c.SubTables)
.Include(c => c.SubTables.Select(b => b.SubSubTables));
}
This works perfect and the JSon returns everything I need, except that I lose the DTO which I desperately need for other aspects of my code. I have rewritten my code in every way I can think of but nothing works. I am pretty sure that this can be done with the DTO but I don't know what it would take to make it work, and as they say "You don't know what you don't know" so hopefully someone here knows.

Answer 1

In Entity Framework 6 (and lower), Include is always ignored when the query ends in a projection, because the Include path can't be applied to the end result. Stated differently, Include only works if it can be positioned at the very end of the LINQ statement. (EF-core is more versatile).
This doesn't help you, because you explicitly want to return DTOs. One way to achieve this is to do the projection after you materialize the entities into memory:
var mainTable = from b in db.MainTables
.Include(b => b.SubTable.Select(e => e.SubSubTable))
.AsEnumerable()
select new MessageDTO()
{
MainTableID = b.MainTableID ,
// eager mapping of all the fields,
SubTables = b.SubTables
};
The phrase, "eager mapping of all the fields" suggests that the projection isn't going to narrow down the SELECT clause anyway, so it won't make much of a difference.
Another way could be to load all SubSubTable objects into the context that you know will be in the MainTables you fetch from the database. EF will populate all SubTable.SubSubTables collections by relationship fixup.

Answer 2

If this works:
public IQueryable<MainTable> GetMainTable()
{
return db.MainTables
.Include(c => c.SubTables)
.Include(c => c.SubTables.Select(b => b.SubSubTables));
}
Then use this one and just add a Select() to the end with a ToList(). Note the IEnumerable in the return type:
public IEnumerable<MainTableDto> GetMainTable()
{
return db.MainTables
.Include(c => c.SubTables)
.Include(c => c.SubTables.Select(b => b.SubSubTables))
.Select(c=> new MainTableDto { SubTables=c.SubTables /* map your properties here */ })
.ToList();
}
Not sure about the types though (at one place you have MainTableDto, at another you mention MessageDto?).