test case for filtering null and not null on relation

[daniel-maegerli]

As discussed, I've written a new test case for checking $null on relations, and changed the existing test case for checking properly on $not:$null on relations. The first test case is failing as expected/described within the issue. Since this is my first time working on this repo (and submitting a PR to an open source project in general), let me know if this is fine.

Issue #538

[ppetzold]

Oh I see. I think the issue is simply that we always LEFT JOIN. So nulled relations will be listed as well. I suppose it must be INNER JOIN to achieve what you expect.

I am currently only working on a sequelize project. Sequelize has the require property on joins. Looking over TypeORM's docs I cannot find the equivalent. Do you know what TypeORMs notation for this is? If so, we can try to imitate.

[Helveg]

@ppetzold I fixed the null queries on relationships by storing that when we use a relationship in a filter that it should be inner joined. On top of that I allow the user to configure the default join method, and the join method per column, since this all converged on the same approach I needed to extend for building the relationships either way.

Oh and and !!! IMPORTANT !

Important

It also fixes an error in the following scenario:

{ filterableColumns: ['home.countCat'] }

Using that filter without including the home in relations used to throw errors like:

column "__root_home_rel_countCat" does not exist

That's fixed now! Whenever you use a relationship in filter it is automatically inner joined :)

This PR is ready for review :)