Unmarshall both flat and nested jsons in golang

Question

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.

Pet is embedded, but it is not anonymous - it's named Pet.

Adrian
– Adrian

2017-06-16 16:50:15 +00:00
Commented Jun 16, 2017 at 16:50 — Adrian
– Adrian, Commented Jun 16, 2017 at 16:50

Adrian · Accepted Answer · 2017-06-16 16:52:31Z

2

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)
}

answered Jun 16, 2017 at 16:52

Adrian

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3 Comments

Kaedys Over a year ago

Ideally, if you can control the program generating the JSON, just use the array annotation exclusively. If the owner only has one pet, then the array only has a single entry, but it's still an array. No reason to have two separate structures to convey the same information when they are not actually logically distinct situations.

Adrian Over a year ago

Agreed. My assumption was that the incoming data was outside their control.

salient Over a year ago

Agree fully, I was looking for a more general pattern to handle denormalized data that needs to be normalized before it goes into a database. The above pattern is elegant

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