Unmarshall both flat and nested jsons in golang

Say I want to be able to handle both nested and unnested jsons of the following form, as in this example:

source_json_1 := `{"owner": "John", "nickname": "Rose", "species": "Dog"}`
source_json_2 := `{"owner": "Doe", "Pet": [{"nickname": "Rose", "species": "Dog"},
			                              {"nickname": "Max", "species": "Cat"}]}`

If I define Pet as an embedded struct I can easily unmarshal it with:

type Owner struct {
	Name string
	Pet
}

type Pet struct {
	NickName	string
	Species		string
}

Resulting in John's pet getting adequately marshalled.

{John {Rose Dog}}
{Doe { }}

But since Pet can actually also be a slice of Pets, Doe's Pets are not correctly unmarshalled. If instead go with

type Owner struct {
	Name string
	Pet  []Pet
}

Then Doe gets marshalled just fine.

{John []}
{Doe [{Rose Dog} {Max Cat}]}

How can I catch both cases?

I'd prefer to marshall it into a slice of Pets in the end, no matter what.

You're looking at two separate data structures, so to unmarshal them with a single struct type, you'd need to account for both:

type Owner struct {
    Name string
    Pet
    Pets []Pet `json:"Pet"`
}

Then, if you want the slice to be authoritative, after you unmarshall, move the embedded to the slice:

// owner := unmarshall blah blah
if owner.Pet != Pet{} {
    owner.Pets = append(owner.Pets, owner.Pet)
}