`@sort` directive semantics

This is a proposal for semantics of a @sort directive. The directive can be used on any property field. It has a direction and optional rank, defined in the schema like this:

directive @sort(direction: String!, rank: Int) on FIELD

The @sort with rank 0 is done first, and then the @sort with rank 1 is used to resolve ties, and so on. If there’s only one @sort, rank can be omitted. If there’s N @sort directives, they must take all ranks from 0 to N - 1.

The @sort directive can be used on fields that are not in the output.

Here’s an example usage:

{
    Animal {
        name @sort(direction: "ascending", rank: 0)
        out_Animal_ParentOf {
            name @sort(direction: "descending", rank: 1)
            uuid @output(out_name: "parent_uuid")
        }
    }
}

The compiler frontend sees whether the provided @sort directives completely determine the output ordering, and includes this information in the output_metadata.

Emitting should be easy to implement, as MATCH, SQL, and Cypher all suport ordering by multiple parameters. I’m not sure about Gremlin, because I couldn’t find anything in its documentation.

[bojanserafimov]

Let’s use enums then:

enum Order {
    ASCENDING,
    DESCENDING,
}

Does the rest of the design sense? Thumbs up this comment if it looks good, so we can start building it.

[bojanserafimov]

Off-topic, but should we be thinking about switching to enums for filter operators as well? I’m not sure = and ones like it are allowed enum values, but maybe there is some opportunity that isn’t immediately obvious…

= is not allowed.

Off-off-topic: If we’re changing the @filter front-end, might be worth thinking about how to express OR.