Support ordering search results by a field.

[poljar]

Tantivy supports ordering the search results from a TopCollector by a field. This used to be done with the TopDocsByField collector and a patch was written to partially utilize this https://github.com/matrix-org/tantivy/commit/85b8d0c7826092b11d81aa1f0b45e5512305f69c.

The latest Tantivy release dropped TopDocsByfield and uses the generic Collector trait instead.

This sadly makes it impossible to modify the mentioned patch since it uses the Any type and down-casting and down-casting trait objects is impossible:

 error: the `downcast_ref` method cannot be invoked on a trait object

There are tricks to work around of this but support for this would need to land in Tantivy.

[bloodbare]

Would be great that search function allows a list of collectors. From the API point of view its better to map Tantivy collectors to Python objects and send them on a list or map each collector to a string so the API would be:

searcher().search(query, 'top')

searcher().search(query, ['top', 'count'])

searcher().search(query, ['top', 'count', { 'person': ['john']}])

or:

top_collector = tantivy.TopDocs(10)
count_collector = tantivy.Count()
facet_collector = tantivy.FacetDocs('/person/john', '/person/bob')
index.searcher().search(query, (top_collector, count_collector, facet_collector))

[fulmicoton]

Yeah or even drop the idea of a collector here, and just add a bunch of options describing what we want.

e.g

• sort_by="bm25"
• count=True -nhits=10 -facets=[]

We most likely won't handle custom collectors in python anyway, so we might as well have a specialized API that is more straightforward to understand.

[poljar]

This has been put on hold since a wrongfully configured schema causes Tantivy to panic. Python users will most likely perceive crashes as bugs, until we can convert the panic into a Python exception this will unlikely be added.

[fulmicoton]

@poljar Agreed. This is a bad behavior for rust too. Let's fix that.

[poljar]

This is now possible, relevant commit 094f8974ea100fbb1eca7aad4078cd650410e51f.