Golang / neoism：查询返回的对象的调用方法导致恐慌

Question

When trying to call a method of a Node object returned by a call to neoism.CypherQuery, I keep getting "invalid memory address or nil pointer dereference" panics. The query returns something (accessing the Data property of the Node works), but calling any method leads to a panic. The methods have the receiver *Node, not Node, but AFAIK that should work nevertheless? Anyway, I have already tried getting a pointer to the object and calling the method on that, but that didn't work either. I'm really stuck here...

Example code to reproduce the problem (needs neoism and go-uuid packages and a Neo4J DB running on localhost):

package main

import (
    "code.google.com/p/go-uuid/uuid"
    "fmt"
    "github.com/jmcvetta/neoism"
)

func main() {
    neo, _ := neoism.Connect("http://localhost:7474/db/data")

    // create a node with a random id
    nodeId := uuid.New()
    _, err := neo.CreateNode(neoism.Props{"NodeId": nodeId})
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println("node created, id", nodeId)

    // find the node by the id
    res := []struct {
        Node neoism.Node `json:"nodes"`
    }{}
    err = neo.Cypher(&neoism.CypherQuery{
        Statement:  `MATCH (nodes {NodeId:{NodeId}}) RETURN nodes`,
        Parameters: neoism.Props{"NodeId": nodeId},
        Result:     &res,
    })
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println("query executed")

    // try to work with the query results
    if len(res) > 0 {
        // get Data -> works
        fmt.Println(res[0].Node.Data)
        // call method -> panics
        err = res[0].Node.SetProperty("TestProp", "TestValue")
        if err != nil {
            fmt.Println(err)
            return
        }
    }
}

Here's the relevant part of the stack trace:

goroutine 1 [running]:
github.com/jmcvetta/neoism.(*entity).SetProperty(0x119abc00, 0x5d3a68, 0x8, 0x5d3a88, 0x9, ...)
        .../src/github.com/jmcvetta/neoism/entity.go:26 +0x104
main.main()
        .../src/nieware/neoprob/neoprob.go:41 +0x4cb

Answer 1

Looking at the source for the SetProperty method: https://github.com/jmcvetta/neoism/blob/master/entity.go#L22

It looks like it comes from an embedded struct and is not actually a method on the Node struct. The embedded entity struct is not a pointer however so it also should not be null.

The stack trace shows that the panic occurs at line 26, because e.Db is not initialized:

resp, err := e.Db.Session.Put(url, &value, nil, &ne)

setting e.Db before calling SetProperty solves the problem:

res[0].Node.Db = neo