mgo ony最后一个JSON条目

Question

I used the code below to store JSON entry in MongoDB, but only the last entry "c2" is stored. What did I do wrong?

package main

import (
    "encoding/json"
    "fmt"
    "labix.org/v2/mgo"
    "labix.org/v2/mgo/bson"
)

func insertEntry(j *map[string]interface{}, entry string) {
    err := json.Unmarshal([]byte(entry), &j)
    if err != nil {
        panic(err)
    }

}

func main() {
    c1 := `{
    "mw" : 42.0922,
    "ΔfH°gas" : {
      "value" : 372.38,
      "units" : "kJ/mol"
    },
    "S°gas" : {
      "value" : 216.81,
      "units" : "J/mol×K"
    },
    "index" : [
      {"name" : "mw", "value" : 42.0922},
      {"name" : "ΔfH°gas", "value" : 372.38},
      {"name" : "S°gas", "value" : 216.81}
    ]
    }`

    c2 := `{
    "name": "silicon",
    "mw": 32.1173,
    "index": [
      {
    "name": "mw",
    "value": 32.1173
      }
    ]
    }`

    m := make(map[string]interface{})

    insertEntry(&m, c1)
    insertEntry(&m, c2)
    chemical := m["ΔfH°gas"].(map[string]interface{})
    fmt.Println("value: ", chemical["value"].(float64))
    fmt.Println("units: ", chemical["units"].(string))

    session, err := mgo.Dial("localhost")

    if err != nil {
        panic(err)
    }
    defer session.Close()

    // Optional. Switch the session to a monotonic behavior.
    session.SetMode(mgo.Monotonic, true)

    c := session.DB("test").C("chemicals")
    err = c.Insert(&m)
    if err != nil {
        panic(err)
    }

    result := &m
    err = c.Find(bson.M{"name": "silicon"}).One(&m)
    if err != nil {
        panic(err)
    }

    fmt.Println(result)
    fmt.Println("mw:", m["mw"])

    fmt.Println("value: ", chemical["value"].(float64))
    fmt.Println("units: ", chemical["units"].(string))

}

Answer 1

It looks like the problem is the way you're building up your map. Your call to json.Unmarshall is taking all of the "top-level" entries in your json and making them keys in your map. In this case, those would be mw, ΔfH°gas, S°gas, index, name, mw, and index.

Note that mw and index are repeated across c1 and c2. That means when you call insertEntry(c2), you're overwriting the mw and index entries. If you print out the contents of your map after each call to insertEntry you can observe this behavior.

There are a few things you could do to avoid this, depending on your needs. You could reformat your json to avoid these conflicts, you could call c.Insert once for each of your c variables (with a fresh map for each one) which would result in different mongodb documents being created for each json chemical definition instead of one big one containing both.