Insight into queries?

traveler_hauptman

I feel like I'm not getting the query system. I get the basics. But then I see something like this:

export function evLength(context is Context, arg is map) returns ValueWithUnits
<snip>
var edges = qEntityFilter(arg.entities, EntityType.EDGE);
    var ownedEdges = qOwnedByBody(arg.entities, EntityType.EDGE);
    return @evLength(context, { "edges" : qUnion([edges, ownedEdges]) }) * meter;

and I get confused.

Why did qEntityFilter not capture the edges owned by entities, making the qUnion necessary?

Does qEntityFilter only work on things that are not owned?

ilya_baran

In this case, arg.entities queries for a mixture of edges and bodies.  qEntityFilter just extracts the edges that are directly queried for.  qOwnedByBody just looks at the bodies -- and then gets their owned edges.  Another way to thing about it is that the result of a qEntityFilter query is always a subset of the subquery passed in -- that's what it means to filter -- so it would be weird if you pass in one body into qEntityFilter and out comes a whole bunch of edges.

I'm not sure if that makes it more or less clear...

traveler_hauptman

That clears it up. It did not occur to me that bodies would be the query passed to qOwnedByBody. I thought it was taking a list of edges and returning edges that were owned by a body.

ilya_baran

Ah, ok.  Actually, *every* edge (and face and vertex) is owned by some body.  You can get that body using qOwnerBody.