Require a specific item in an array - json

Question

I have the following json object:
{
"my_items": [
{ "a": "primary", n: 1 },
{ "b": "secondary", n: 2 },
{ "b": "secondary", n: 3 }
]
}
All items in the my_items list are expected to be unique. Now, I need to validate the entire json object with the following rule:
it may contain zero or multiple items with "type": "secondary", but it absolutely must contain one and only one item with "type": "primary".
How can this be expressed using latest json-schema?
I'm come up with the following:
var schema = {
"definitions": {
"primary_item": {
"type": "object",
"properties": {
"a": {
"type":"string",
"enum":["primary"]
}
}
},
"secondary_item": {
"type": "object",
"properties": {
"b": {
"type": "string",
"enum":["secondary"]
}
}
}
},
"type": "object",
"properties": {
"my_items": {
"type": "array",
"minItems": 1,
"contains": {"$ref": "#/definitions/primary_item"},
"items": {
"anyOf": [
{"$ref": "#/definitions/primary_item"},
{"$ref": "#/definitions/secondary_item"}
],
"additionalProperties": false
}
}
},
"additionalProperties": false
};
var validate = ajv.compile(schema);
test({
"my_items": [
{"a": "primary"},
{"b": "secondary"},
{"b": "secondary"}
]
});
But the tests are failing, with the following errors:
Invalid: data.my_items[0] should NOT have additional properties, data.my_items[1] should NOT have additional properties, data.my_items[2] should NOT have additional properties

Answer 1

There is no way in JSON Schema to assert that an array contains one and only one of something. You can assert there is at least one, but that's the best you can do. The closest you can get is to require that the primary_item is the first element in the array.
{
"type": "object",
"properties": {
"my_items": {
"type": "array",
"items": [
{"$ref": "#/definitions/primary_item"}
],
"additionalItems": {"$ref": "#/definitions/secondary_item"}
}
},
"additionalProperties": false,
"definitions": {
"primary_item": {
"type": "object",
"properties": {
"a": { "enum":["primary"] }
},
"additionalProperties": false
},
"secondary_item": {
"type": "object",
"properties": {
"b": { "enum":["secondary"] }
},
"additionalProperties": false
}
}
}
EDIT
Responding to the comment
I wonder if one and only one could be expressed using two concepts: at least one + unique.
Yes. If your array items are unique you can do the following.
{
"type": "object",
"properties": {
"my_items": {
"type": "array",
"items": { "$ref": "#/definitions/my_item" },
"allOf": [{"$ref": "#/definitions/contains_primary_item"}],
"uniqueItems": true
}
},
"additionalProperties": false,
"definitions": {
"my_item": {
"type": "object",
"properties": {
"a": { "type": "string" }
},
"additionalProperties": false
},
"primary_item": {
"type": "object",
"properties": {
"a": { "enum":["primary"] }
},
"additionalProperties": false
},
"contains_primary_item": {
"not": {
"items": {
"not": { "$ref": "#/definitions/primary_item" }
}
}
}
}
}