Restheart Aggregation Framework - 可选的Avars？

Question

We have an aggregation query in RestHeart which works well.

In the match section, we allow users to specify a specific document Id, to return just that document:

                "_$match" : {
                    "_id" : {
                        "_$var" : "n"
                    },

However, we've also like users to be able to omit this, and get all documents in the collection.

Is there any way to make avars optional, or to specify a wildcard for that avar ? Currently if the user omits the avar, they get an error. And setting the avar to null or an empty string literally matches those values.

I guess another alternative would be if there can be conditional logic in the aggregation query properties -- not sure if this is possible.

Thanks

Tom

Answer 1

I have found a simpler solution. The aggregation query in the _properties collection can be kept as-is, but instead, a wildcard can be specified in the GET request. Here is an example:

GET https://test.restheartapi.com:4443/DataBase/collection/_aggrs/items?avars={"n":/.*/, "m":/.*/ }

... So although it isn't possible to make avars optional in RestHeart, it's still possible to specify a wildcard as the value.

I'm not sure if there's a small performance hit by using a regular expression like this, but it seems to be the only simple way to do this for now.

Answer 2

You can have a conditional aggregation using the $cond operator

Evaluates a boolean expression to return one of the two specified return expressions.

The $cond expression has one of two syntaxes:

{ $cond: { if: <boolean-expression>, then: <true-case>, else: <false-case-> } }

Or:

{ $cond: [ <boolean-expression>, <true-case>, <false-case> ] }