AJV schema validation error - json

Question

I have the input json like below,
{"contents":[{"type":"field"},{"type":"field","itemId":"594b9980e52b5b0768afc4e8"}]}
the condition is,
if the type is 'field', then 'itemId' should be the required field
and if the type is 'fieldGroup' or 'subSection', then 'itemId' is optional
This is the Json Schema I tried and its not working as expected,
"type": "object",
"additionalProperties": false,
"properties" : {
"contents" : {
"type" : "array",
"items": {"$ref": "#displayItem" }
}
},
"definitions": {
"displayItem" : {
"id": "#displayItem",
"type": "object",
"items": {
"anyOf": [
{"$ref": "#fieldType"},
{"$ref": "#fieldGroupSubSectionType"}
]
}
},
"fieldType" : {
"id": "#fieldType",
"type": "object",
"additionalProperties": false,
"properties": {
"itemId": {
"type": "string"
},
"type": {
"type": "string",
"enum": ["field"]
}
}
},
"fieldGroupSubSectionType" : {
"id": "#fieldGroupSubSectionType",
"type": "object",
"additionalProperties": false,
"properties": {
"itemId": {
"type": [ "string", "null" ]
},
"type": {
"type": "string",
"enum": [
"fieldGroup",
"subSection"
]
}
}
}
}
Any help / workaround with Sample Json Schema to achieve the above use case is appreciated.

Answer 1

If I understand the description of what you want correctly, then the json example you provide is not valid since it has a type: "field" but does not have an "itemId" property.
Assuming that is true. Instead of using
type: ["string", null]
use the required property.
I changed your schema a bit, instead of having separate definitions I inlined them, but other than that (and the use of required) is the same:
{
"type": "object",
"additionalProperties": false,
"properties": {
"contents": {
"type": "array",
"items": {
"anyOf": [
{
"type": "object",
"additionalProperties": false,
"properties": {
"itemId": {
"type": "string"
},
"type": {
"type": "string",
"enum": [
"field"
]
}
},
"required": [
"itemId"
]
},
{
"type": "object",
"additionalProperties": false,
"properties": {
"itemId": {
"type": "string"
},
"type": {
"type": "string",
"enum": [
"fieldGroup",
"subSection"
]
}
}
}
]
}
}
}
}

Answer 2

Here is your answer with a little cleanup for best practices and style. The trick is that you need to use implication "a implies b <=> (not a) or b". In this case you have "type = field implies itemId is required <=> type is not field or itemId is required".
{
"type": "object",
"properties": {
"contents": {
"type": "array",
"items": { "$ref": "#/definitions/displayItem" }
}
},
"definitions": {
"displayItem": {
"type": "object",
"properties": {
"itemId": { "type": "string" },
"type": { "enum": ["field", "fieldGroup", "subSection"] }
},
"anyOf": [
{ "not": { "$ref": "#/definitions/fieldType" } },
{ "required": ["itemId"] }
]
},
"fieldType": {
"properties": {
"type": { "enum": ["field"] }
}
}
}
}