Feature: Filter/Having on included collections

[johannes-z]

As of now, includes cannot be filtered upon. You need a sub-query with a join to filter.
For example, given the official example that lists projects and issues in a single createLiveQueryCollection, you cannot filter out projects that do not have issues.

import { createLiveQueryCollection, eq, toArray } from '@tanstack/db'

const projectsWithIssues = createLiveQueryCollection((q) =>
  q.from({ p: projectsCollection }).select(({ p }) => ({
    id: p.id,
    name: p.name,
    issues: toArray(
      q
        .from({ i: issuesCollection })
        .where(({ i }) => eq(i.projectId, p.id))
        .select(({ i }) => ({
          id: i.id,
          title: i.title,
        })),
    ),
  })),
)

If I want to filter projects without issues, I have to do this:

import { createLiveQueryCollection, eq, toArray } from '@tanstack/db'

const projectsWithIssues = createLiveQueryCollection((q) => {
  const projectIdsWithIssues = q
    .from({ issueRef: issuesCollection })
    .select(({ issueRef }) => ({
      projectId: issueRef.projectId,
    }))
    .distinct()

  return q
    .from({ issueProject: projectIdsWithIssues })
    .innerJoin({ p: projectsCollection }, ({ issueProject, p }) =>
      eq(issueProject.projectId, p.id),
    )
    .select(({ p }) => ({
      id: p.id,
      name: p.name,
      issues: toArray(
        q
          .from({ i: issuesCollection })
          .where(({ i }) => eq(i.projectId, p.id))
          .select(({ i }) => ({
            id: i.id,
            title: i.title,
          })),
      ),
    }))
})

I'm not aware of a cleaner/shorter way to write this. It would be nice if we could use .fn.having or .fn.where on the the included collection, for example:

import { createLiveQueryCollection, eq, toArray } from '@tanstack/db'

const projectsWithIssues = createLiveQueryCollection((q) =>
  q.from({ p: projectsCollection }).select(({ p }) => ({
    id: p.id,
    name: p.name,
    issues: toArray(
      q
        .from({ i: issuesCollection })
        .where(({ i }) => eq(i.projectId, p.id))
        .select(({ i }) => ({
          id: i.id,
          title: i.title,
        })),
    ),
  }))
  .fn.having(({ $selected }) => $selected.issues.length > 0), // or .fn.where ...
)

The subquery workaround works, but it's wordy, especially with bigger queries. Additionally, all materialized collections need a unique name (issueRef instead of issue in the example above). For bigger queries this gets super wordy.