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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