Concat 2 mongodb collections - mongodb

Question

I'm trying to combine 2 collections into one (not join). I have 2 databases with same collections and collection structure.
As example:
Collection test1 db1:
{
"_id" : ObjectId("574c339b3644a65b36e77359"),
"appName" : "App1",
"customerId" : "Client1",
"environment" : "PROD",
"methods" : []
}
Collection test2 db2:
{
"_id" : ObjectId("574c367d627b45ef0abc00e5"),
"appName" : "App2",
"customerId" : "Client2",
"environment" : "PROD",
"methods" : []
}
I'm trying to create the following:
One collection test db, where the documents will be merged from test1 and test2 but not one with each other. What would be the proper way to achieve this?
{
"_id" : ObjectId("574c339b3644a65b36e77359"),
"appName" : "App1",
"customerId" : "Client1",
"environment" : "PROD",
"methods" : []
},
{
"_id" : ObjectId("574c367d627b45ef0abc00e5"),
"appName" : "App2",
"customerId" : "Client2",
"environment" : "PROD",
"methods" : []
}
The complexity is that ID are referenced in other collection of mongo.

Answer 1

the fastest way will be to create a dump (using mongodump) and restore them at once (example is using windows paths).
mongodump --db test1 --collection test1 --out c:\dump\test1
mongodump --db test2 --collection test2 --out c:\dump\test2
mongorestore --db test3 --collection test3 c:\dump\test1
mongorestore --db test3 --collection test3 c:\dump\test2