CSHARP-5396: Add support for arbitrary join inner expression arguments in LINQ

[kerem-acer]

This PR add support for arbitrary join inner expression arguments in LINQ.

Problem

The MongoDB LINQ translator currently throws an exception when attempting to join a foreign collection queryable that includes additional query operations (like .Where()).

Example of the Issue

When trying to join two collections with filtering conditions, the following code throws an exception:

// Collection setup and mapping code...

var pagesWithContainers = await (from page in pages.AsQueryable().Where(x => x.TenantId == 1)
    join container in pageContainers.AsQueryable().Where(x => x.TenantId == 1) 
    on page.Id equals container.PageId into containers
    select new
    {
        page.Id,
        page.Title,
        Containers = containers
    }).ToListAsync();

The code only works if you remove the .Where() clause from pageContainers. This PR adds support for such filtering operations on joined collections.

Current Error

Without this PR, the above code throws the following exception:

MongoDB.Driver.Linq.ExpressionNotSupportedException: Expression not supported: [...] 
because inner expression must be a MongoDB IQueryable against a collection.

Todos

• Write comprehensive tests

[kerem-acer]

Even though now it doesn't throw exceptions, Where()'s of foreign collection queryable are ignored. Probably more work is needed to get this job done.

As I understand, in JoinMethodToPipelineTranslator.Translate method line 66, innerExpression(foreign queryable expression) is used only for collectionName. So JoinMethodToPipelineTranslator.Translate does not respect the content of innerExpression.

[rstam]

You are correct, we expect the inner argument to Join to be just a way to specify the inner collection, so we expect this:

Join(inner.AsQueryable(), ...)

and not:

Join(inner.AsQueryable().Where(...), ...)

This is by design.

I'm not sure how much work it would be, or if it is even possible, to support arbitrary inner arguments. In any case the first step would be to figure out if there even is an MQL translation that would return the desired results.

[kerem-acer]

Okay. Now, this this is a new feature rather than a bug. I will investigate and try to implement this feature this week. I will update you here.

Also, should I update the title and description of this PR and Jira issue? The scope of this PR has changed.

[kerem-acer]

I implemented this feature and changed content of this PR.

fyi @rstam

[kerem-acer]

Should we create a new TranslationContext for innerExpression?