How to insert a struct with nil field to mongo?

Question

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. A nil slice == a zero length slice, but a nil pointer to a slice does not. — Adrian
– Adrian, Commented Aug 11, 2017 at 14:49
My script runs on crontab and it compares values on Mongo and values from API if they have changed and need to be updated in mongo. But the comparison cannot happen corectly if some fields from null become zero value. I use DeepEqual to compare and it considers nil ant [] different things (of course it is supposed to). Thats Why I need to have exact data type — SokIsKedu
– SokIsKedu, Commented Aug 12, 2017 at 16:48
@Adrian Your solution is exactly what I was looking for, it solves my problem. — SokIsKedu
– SokIsKedu, Commented Aug 16, 2017 at 14:11

Adrian · Accepted Answer · 2017-08-16 14:13:49Z

1

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.

answered Aug 16, 2017 at 14:13

Adrian

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