Marten: Add support for including child collection objects

[kamranayub]

For post-1.0, add support for allowing Include on child collection objects:

var items = new List<Item>();
session.Query<User>().Include(u => u.Collection.Select(c => c.ItemId), items);

[jokokko]

Maybe something like

SELECT * FROM public.mt_doc_issuelist as il
INNER JOIN public.mt_doc_issue i on (il.Data->'Issues') @> to_jsonb(t.Id)
-- or from Marten object serialization (in this case yielding equal execution plans)
SELECT * FROM public.mt_doc_issuelist as il
INNER JOIN public.mt_doc_issue i on (il.Data->'Issues') @> (t.Data->'Id')

C# public class IssueList { public List<Guid> Issues { get; set; } ... } query.Query<IssueList>().Include<Issue>(x => x.Issues, i => issuesFromDb.Add(i))
By quick glance, the current implementation of IncludeJoin does not seem to favour this, though.

[jeremydmiller]

Maybe this would help do it in one call: https://tech.npras.in/postgres-recursive-query/

[jokokko]

Well, I hope to take a look at this in the near future. But just by gut feeling & past memory, I can see #834 relating. So addressing how LINQ expressions are evaluated, exposed and translated in the pipeline might provide better & more generalizable benefits overall, instead of attacking the problems case-by-case with a very spefic constructs built around the existing pipeline.

[jeremydmiller]

@jokokko Are you up for helping out on this if we really do take it on? Gonna put it off until I clear up a bit and get over the flu first though;-)

[jokokko]

@jeremydmiller Yeah I hope to help. Overall, I have to go through code to identify scenarios with dance around queries that the current LINQ provider cannot translate.

[jeremydmiller]

Believe it or not we're finally working on this again. This time I want to consider using Common Table Expressions in Postgresql to pull off the fetching

[jeremydmiller]

Closed by https://github.com/JasperFx/marten/commit/c0a86fdb6fb61251aea4754afda89345b90b5cbd