Allow aggregation of queryables across collections

[gadomski]

It can be useful to find the set of queryables available across collections. Here's one possible usage:

client = Client.open("https://pystac-client.test")

# GET /queryables
_: Set[str] = client.queryables()

# GET /queryables/collection-a/queryables
# GET /queryables/collection-b/queryables
# Put all queryables in a set and return
_: Set[str] = client.queryables(["collection-a", "collection-b"])

There's a question to resolve around whether queryables() should hit /queryables or do an aggregation of all collections.

[jsignell]

Is the response different from get_queryables? If we want allow aggregation maybe we could add a recursive option to get_queryables?

[gadomski]

Ah no, thanks! I forgot that we just added that :-). The primary motivating use-case was around "I have this subset of collections that I want to query against", so just recursive wouldn't be enough -- we'll want to specify collection ids. But maybe:

client.get_queryables(collections=["a", "b"])

that would hit /collections/a/queryables and /collections/b/queryables and union the queryables together.

[jsignell]

Yeah and that feels more ergonomic than something like this?

set(client.get_collection(c).get_queryables() for c in ["a", "b"])

[ircwaves]

The response from get_queryables has "$id", "$schema", "properties" as the keys.

1. $schema seems straight forward, as that is always the same.
2. $id is less clear to me. I default to wanting to drop it.
3. properties seems like what @jsignell suggested could be implemented by successive .update calls, but this could be erroneous if a parameter has different values in separate collections, and only the last value would be in there.

@gadomski -- does that fit the bill for this?

[gadomski]

$schema seems straight forward, as that is always the same.

Hopefully, right? Probably worth assert-ing just in case.

$id is less clear to me. I default to wanting to drop it.

Yeah, drop it.

if a parameter has different values in separate collections, and only the last value would be in there.

In this case I think we should drop the entry. If the $ref is different for the same queryable key, then we can't reliably query on that key IMO.

What to do about additionalProperties? I think we should assert that it's the same for every collection's queryables, and throw an error if it isn't.