Loop over collections in mongodb and decode to various go structs - mongodb

Question

I have a series of collections in a mongodb instance where I want to pull all the collections and decode all documents in the collections so I can mutate them as I go.
I am using go to list the collection names:
...
collections, err := db.ListCollectionNames(context.TODO(), bson.M{})
if err != nil {
log.Fatalf("Failed to get coll names: %v", err)
}
for _, coll := range collections {
collection := db.Collection(coll)
...
This works flawlessly and the next step would be to open a cursor to each collection and loop over the cursor and call decode on each object to its corresponding type.
I have defined all the types in a types package:
type Announcements struct {
Id string `bson:"_id"`
Content string `bson:"content"`
Title string `bson:"title"`
DateModified time.Time `bson:"dateModified,omitempty"`
ModifiedBy string `bson:"modifiedBy,omitempty"`
}
The issue is that I cant define the variable to be used for decoding dynamically.
This code would need to be repeated for every single collection
var obj Announcements // This would need to be dynamic
for cursor.Next(context.Background()) {
err := cursor.Decode(&obj)
if err != nil {
log.Fatal(err)
}
log.Printf("%#v\n", obj)
}
Is there a way to do this without repeating this many times? I realize a dynamically typed language would be better for this. Wanted to ask before I migrate the work to a scripting language.
I have tried using the reflect package and switch statements to instantiate the variable dynamnically and using the interface{} type but that force the decode to use bson types and not my defined structs.
-- edit --
I have tried to use a map to link collection names to types but to no avail.
var Collections = map[string]interface{}{
"announcements": Announcements{},
"badges": Badges{},
"betaUser": BetaUser{},
"bonusLearningItems": BonusLearningItems{},
"books": Books{},
...
}
So that when I use the obj var I tried an assignment like:
var obj types.Collections[coll]
hoping that would give me a variable of type Announcement if I was had looped to the announcements collection. but when I call decode it returns bson types.
I need to dynamically define the obj variable type.

Answer 1

There are many ways you can do this. One of them is by using a callback function:
func Process(cursor *mongo.Cursor, cb func(), out interface{}) {
for cursor.Next(context.Background()) {
err := cursor.Decode(out)
if err != nil {
log.Fatal(err)
}
cb()
log.Printf("%#v\n", obj)
}
}
Then define a map:
type collHandler struct {
New func() interface{}
Process func(interface{})
}
var collections=map[string]func() interface{} {
"announcements": collHandler{
New: func() interface {} {return &Announcement{}},
Process: func(data interface{}) {
processAnnouncements(data.(*Announcement))
},
},
...
}
And then you can:
handler:=collections[collectionName]
obj:=handler.New()
process(cursor,func() {
handler.Process(obj)
},
&obj)
This way, you can place the iteration into a common function, and use closures to deal with type specific logic.