How do I convert from a slice of interface{} to a slice of my struct type in Go?

Question

func GetFromDB(tableName string, m *bson.M) interface{} {
    var (
        __session *mgo.Session = getSession()
    )

    //if the query arg is nil. give it the null query
    if m == nil {
        m = &bson.M{}
    }

    __result := []interface{}{}
    __cs_Group := __session.DB(T_dbName).C(tableName)
    __cs_Group.Find(m).All(&__result)

    return __result
}

call

GetFromDB(T_cs_GroupName, &bson.M{"Name": "Alex"}).([]CS_Group)

runtime will give me panic:

panic: interface conversion: interface is []interface {}, not []mydbs.CS_Group

how convert the return value to my struct?

score 3 · Answer 1 · answered Oct 01 '14 at 12:12

3

You can't automatically convert between a slice of two different types – that includes []interface{} to []CS_Group. In every case, you need to convert each element individually:

s := GetFromDB(T_cs_GroupName, &bson.M{"Name": "Alex"}).([]interface{})
g := make([]CS_Group, 0, len(s))
for _, i := range s {
    g = append(g, i.(CS_Group))
}

answered Oct 01 '14 at 12:12

djd

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very thanks.[]interface{}{} is type in type. i before think go can convert type in array only one step. I was wrong. – qwe520liao Oct 01 '14 at 13:32
but i use the method. will show the error: [panic: interface conversion: interface is bson.M, not mydbs.CS_Group] – qwe520liao Oct 01 '14 at 14:14

Simon Fox · Accepted Answer · 2014-10-01T17:16:13.910

You need to convert the entire hierarchy of objects:

rawResult := GetFromDB(T_cs_GroupName, &bson.M{"Name": "Alex"}).([]interface{})
var result []CS_Group
for _, m := range rawResult {
   result = append(result, 
       CS_Group{
         SomeField: m["somefield"].(typeOfSomeField),
         AnotherField: m["anotherfield"].(typeOfAnotherField),
       })
}

This code is for the simple case where the type returned from mgo matches the type of your struct fields. You may need to sprinkle in some type conversions and type switches on the bson.M value types.

An alternate approach is to take advantage of mgo's decoder by passing the output slice as an argument:

func GetFromDB(tableName string, m *bson.M, result interface{}) error {
    var (
        __session *mgo.Session = getSession()
    )
    //if the query arg is nil. give it the null query
    if m == nil {
        m = &bson.M{}
    }
    __result := []interface{}{}
    __cs_Group := __session.DB(T_dbName).C(tableName)
    return __cs_Group.Find(m).All(result)
}

With this change, you can fetch directly to your type:

 var result []CS_Group
 err := GetFromDB(T_cs_GroupName, bson.M{"Name": "Alex"}, &result)

See also: FAQ: Can I convert a []T to an []interface{}?

How do I convert from a slice of interface{} to a slice of my struct type in Go?

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