Across an array of data, how to require a particular value appear once?

I am am trying to generate data such as the following

[{
    "id": 1,
    "company": "Doogle",
    "contacts": [{ 
        "primary": true,
        "first_name": "John",
        "last_name": "Doe"
    }]
},
{
    "id": 2,
    "company": "Bahoo",
    "contacts": [{ 
        "primary": false,
        "first_name": "Peter",
        "last_name": "Drake"
    },
    { 
        "primary": true,
        "first_name": "Jane",
        "last_name": "Smith"
    }]
}]

Contacts is an array of objects, how do I enforce the rule that one and only one contact can be primary and there must be at least one primary contact?