How to insert a struct with nil field to mongo?

I am trying to insert a struct to mongo. Firstly I get the data from an API as JSON and assign the data to a struct. Some fields might be nil. After that I insert the struct to mongoDB. So the problem I get is that when inserted, all the fields are initialized. For example I have a struct like this:

type VirtualMachine struct {
VirtualMachineID          utils.SUUID               `bson:"VirtualMachineID"`
Cdroms                    []*VM.VirtualMachineCdrom `bson:"Cdroms"`
CpuAllocatedMHz           int                       `bson:"CpuAllocatedMHz"`
Name                      string                    `bson:"Name"`
}

If I get Json data like this

{
"VirtualMachineID":'16as4df663a',
"Cdroms":null,
"CpuAllocatedMHz":1666,
"Name":'VMName'
}

after I put it to mongo, the null field becomes an empty array. I need to avoid that. 'omitempty' did not help because it skips the field as well if the provided field happens to be an empty array and not null.

Firstly I thought it was because of the pointers, but later I found that the same happens to all data types. Shortly, if its nil, mgo converts it to its zero value.

I think I am missing something here, because it would be weird if mgo converts all nil values to their zero values by design.

Try *[]*VM.VirtualMachineCdrom (or just *[]VM.VirtualMachineCdrom if you don't actually need the elements to be pointers). A nil slice == a zero length slice, but a nil pointer to a slice does not.