Getting pagination to work with one to many join - servicestack

Question

I'm currently working on a database with several one-to-many and many-to-many relationships and I am struggling getting ormlite to work nicely.
I have a one-to-many relationship like so:
var q2 = Db.From<GardnerRecord>()
.LeftJoin<GardnerRecord, GardnerEBookRecord>((x, y) => x.EanNumber == y.PhysicalEditionEan)
I need to return a collection of ProductDto that has a nested list of GardnerEBookRecord.
Using the SelectMult() technique it doesn't work because the pagination breaks as I am condensing the left joined results to a smaller collection so the page size and offsets are all wrong (This method: How to return nested objects of many-to-many relationship with autoquery)
To get the paging right I need to be able to do something like:
select r.*, count(e) as ebook_count, array_agg(e.*)
from gardner_record r
left join gardner_e_book_record e
on r.ean_number = e.physical_edition_ean
group by r.id
There are no examples of this in the docs and I have been struggling to figure it out. I can't see anything that would function like array_agg in the Sql object of OrmLite.
I have tried variations of:
var q2 = Db.From<GardnerRecord>()
.LeftJoin<GardnerRecord, GardnerEBookRecord>((x, y) => x.EanNumber == y.PhysicalEditionEan)
.GroupBy(x => x.Id).Limit(100)
.Select<GardnerRecord, GardnerEBookRecord>((x, y) => new { x, EbookCount = Sql.Count(y), y }) //how to aggregate y?
var res2 = Db.SelectMulti<GardnerRecord, GardnerEBookRecord>(q2);
and
var q2 = Db.From<GardnerRecord>()
.LeftJoin<GardnerRecord, GardnerEBookRecord>((x, y) => x.EanNumber == y.PhysicalEditionEan)
.GroupBy(x => x.Id).Limit(100)
.Select<GardnerRecord, List<GardnerEBookRecord>>((x, y) => new { x, y });
var res = Db.SqlList<object>(q2);
But I can't work out how to aggregate the GardnerEBookRecord to a list and keep the paging and offset correct.
Is this possible? Any workaround?
edit:
I made project you can run to see issue:
https://github.com/GuerrillaCoder/OneToManyIssue
Database added as a docker you can run docker-compose up. Hopefully this shows what I am trying to do

Answer 1

Npgsql doesn't support reading an unknown array or records column type, e.g array_agg(e.*) which fails with:
Unhandled Exception: System.NotSupportedException: The field 'ebooks' has a type currently unknown to Npgsql (OID 347129).
But it does support reading an array of integers with array_agg(e.id) which you can query instead:
var q = #"select b.*, array_agg(e.id) ids from book b
left join e_book e on e.physical_book_ean = b.ean_number
group by b.id";
var results = db.SqlList<Dictionary<string,object>>(q);
This will return a Dictionary Dynamic Result Set which you'll need to combine into a distinct id collection to query all ebooks referenced, e.g:
//Select All referenced EBooks in a single query
var allIds = new HashSet<int>();
results.Each(x => (x["ids"] as int[])?.Each(id => allIds.Add(id)));
var ebooks = db.SelectByIds<EBook>(allIds);
Then you can create a dictionary mapping of id => Ebook and use it to populate a collection of ebooks entities using the ids for each row:
var ebooksMap = ebooks.ToDictionary(x => x.Id);
results.Each(x => x[nameof(ProductDto.Ebooks)] = (x["ids"] as int[])?
.Where(id => id != 0).Map(id => ebooksMap[id]) );
You can then use ServiceStack AutoMapping Utils to convert each Object Dictionary into your Product DTO:
var dtos = results.Map(x => x.ConvertTo<ProductDto>());