I would guess this is because when iter.Next is given a nil bson.M is
needs to make a new value to fill (which from your output is a map).
When it is handed a bson.M that already has a map in it, it can use
that. Remember that bson.M is just interface{} and maps are pointer-
like.
On Mon, 2016-10-24 at 11:56 -0700, Diego Medina wrote:
> Hi (sorry for the cryptic subject, I'm just not sure what to even
> call
> this):
>
>
> I think this is not strictly a mgo question, I have this simplified
> function that gets documents from a mongo instance using Iter(), I
> then
> loop through the results, append them to a slice []interface{} but
> the
> resulting new slice has the last element repeated N times, where N is
> the
> number of item I got from the call to Iter().
>
> In code on gist:
>
> https://gist.github.com/fmpwizard/a1b4cc246b5875ff6bd1fbcd71940703
>
> pasted here:
>
>
> func TestBatchInsert1(t *testing.T) {
> c := session.DB("testing").C("delete")
> c.DropCollection()
>
> c.Insert(bson.M{
> "name": "AAAAAAA",
> "num": 1,
> })
>
> c.Insert(bson.M{
> "name": "BBBBBBBB",
> "num": 2,
> })
>
> iter := c.Find(nil).Iter()
> //var doc interface{} //if I use this line instead of a bson.M, the
> "issue"
> doesn't happen
> var doc bson.M
> var docs []interface{}
>
> for iter.Next(&doc) {
> fmt.Printf("Pre append\n\n %+v\n\n", docs)
> docs = append(docs, doc)
> fmt.Printf("Post append\n\n %+v\n\n", docs)
> //doc = nil //This **fixes** it but why?
> }
> fmt.Printf("Final\n\n %+v\n\n", docs)
>
> err := iter.Close()
> if err != nil {
> fmt.Printf("Failed to close iter during copy: %s", err)
> }
> }
>
>
> the output of all that is:
>
> Pre append
>
> []
>
> Post append
>
> [map[_id:ObjectIdHex("580e5758ad817545b924a2c1") name:AAAAAAA
> num:1]]
>
> Pre append
>
> [map[_id:ObjectIdHex("580e5758ad817545b924a2c2") name:BBBBBBBB
> num:2]]
>
> Post append
>
> [map[_id:ObjectIdHex("580e5758ad817545b924a2c2") name:BBBBBBBB
> num:2]
> map[_id:ObjectIdHex("580e5758ad817545b924a2c2") name:BBBBBBBB num:2]]
>
> Final
>
> [map[_id:ObjectIdHex("580e5758ad817545b924a2c2") name:BBBBBBBB
> num:2]
> map[_id:ObjectIdHex("580e5758ad817545b924a2c2") name:BBBBBBBB num:2]]
>
>
> ===========================
>
> note how the second time we print "Pre append" the docs slice has is
> the
> second item in it, instead of the first one:
>
> [map[_id:ObjectIdHex("580e5758ad817545b924a2c2") name:BBBBBBBB
> num:2]]
>
> as if I appended a pointer to the docs instead the actual value of
> doc.
>
> I found two ways to avoid this issue, one is to uncomment
>
> //doc = nil //
>
> after calling append, I just don't understand why I need to reset it.
>
> The other is, instead of working with
>
> var doc bson.M
> var docs []interface{}
>
> I can use
>
> var doc interface{}
> var docs []interface{}
>
> but using
>
> var doc bson.M
> var docs []bson.M
>
> also shows the **issue**
>
> Thanks.
>
> Diego
>
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