英文:
Golang/neoism: calling methods of objects returned by query leads to panic
问题
尝试调用neoism.CypherQuery返回的Node对象的方法时，我一直收到"invalid memory address or nil pointer dereference"的错误。查询返回了一些结果（可以访问Node的Data属性），但调用任何方法都会导致错误。这些方法的接收者是*Node，而不是Node，但据我所知，这应该仍然有效。无论如何，我已经尝试获取对象的指针并在其上调用方法，但也没有成功。我真的被困住了...
以下是重现问题的示例代码（需要neoism和go-uuid包以及在本地主机上运行的Neo4J数据库）：
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")
// 创建一个具有随机id的节点
nodeId := uuid.New()
_, err := neo.CreateNode(neoism.Props{"NodeId": nodeId})
if err != nil {
fmt.Println(err)
return
}
fmt.Println("node created, id", nodeId)
// 根据id查找节点
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")
// 尝试处理查询结果
if len(res) > 0 {
// 获取Data - 可行
fmt.Println(res[0].Node.Data)
// 调用方法 - 引发错误
err = res[0].Node.SetProperty("TestProp", "TestValue")
if err != nil {
fmt.Println(err)
return
}
}
}
以下是堆栈跟踪的相关部分：
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
英文:
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
答案1
得分: 2
看一下SetProperty方法的源代码：https://github.com/jmcvetta/neoism/blob/master/entity.go#L22
看起来它来自一个嵌入的结构体，实际上并不是Node结构体上的一个方法。嵌入的entity结构体不是一个指针，所以它也不应该为空。
堆栈跟踪显示，恐慌发生在第26行，因为e.Db没有初始化：
resp, err := e.Db.Session.Put(url, &value, nil, &ne)
在调用SetProperty之前设置e.Db可以解决这个问题：
res[0].Node.Db = neo
英文:
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
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