Inconsistency of implementation in Redpath

[miaoxw]

Recently I am making some exploration for further use of libredfish. I tried to call getPayloadByPath after reading documents and README page,

In such an environment that contains one member of System, the redPath /Systems returns the payload below:

{
    "@odata.context":"/redfish/v1/$metadata#ComputerSystemCollection.ComputerSystemCollection",
    "@odata.id":"/redfish/v1/Systems",
    "@odata.type":"#ComputerSystemCollection.ComputerSystemCollection",
    "Description":"Collection of Computer Systems",
    "Members":[
        {
            "@odata.id":"/redfish/v1/Systems/System.Embedded.1"
        }
    ],
    "Members@odata.count":1,
    "Name":"Computer System Collection"
}

But when I tried to access its only member, getPayloadByPath returns NULL for error when I passed the redPath /Systems/Members[1]. In an occasional attempt, I got the right result with /Systems/Members[0].

According to the freshly released Redpath Specification, index is 1-based instead of 0-based in conventional programming language (JSON in this issue which caused the problem). This is also specially noted in the example query at line 102:

* /Chassis[1]: Returns the first Chassis resource

Therefore I think implementation of redpath in the current version has conflicts with specification.

#iwork4dell

[mraineri]

We might want to consider making an errata to the Redpath spec; the spec was intended to be based off libredfish, and 0-based indexing into JSON arrays seems more natural. Will need to discuss this others.

[dchanman]

Errata works for Google, it turns out our current implementation was also 0-indexing.

[miaoxw]

Our Redpath Specification says Redpath is based on XPath and XPath is 1-based. However, keeping the library as-is is indeed an option too. Either implmementation makes sense. I think it is OK as long as lib and spec are consistent.

[mraineri]

We've published an updated version of the RedPath spec to clarify the indexing is 0-based: https://www.dmtf.org/sites/default/files/standards/documents/DSP0285_1.0.1.pdf

Selects the index number JSON entity from an array or object. For arrays, the index is 0-based.

I also see the README isn't consistent either; I'll make a fix for this.

[mraineri]

Just updated the README:

/Chassis[0] - Will return the first Chassis instance

@miaoxw if everything looks good at this point, I'd like to close the issue.

[miaoxw]

@mraineri Hi Mike, it looks good to me now. You are safe to close this issue :)