How to generate a JSON Schema? - node.js

Question

I would like to generate a schema from a json object.
var GenerateSchema = require('generate-schema')
var schema = GenerateSchema.json(request.body);
Request.Body
{
"type": "FeatureCollection",
"features": [
{
"type": "Feature",
"geometry": {
"type": "Point",
"coordinates": [
102,
0.5
]
},
"properties": {
"prop0": "value0"
}
},
{
"type": "Feature",
"geometry": {
"type": "LineString",
"coordinates": [
[
102,
0
],
[
103,
1
],
[
104,
0
],
[
105,
1
]
]
},
"properties": {
"prop0": "value0",
"prop1": 0
}
},
{
"type": "Feature",
"geometry": {
"type": "Polygon",
"coordinates": [
[
[
100,
0
],
[
101,
0
],
[
101,
1
],
[
100,
1
],
[
100,
0
]
]
]
},
"properties": {
"prop0": "value0",
"prop1": {
"this": "that"
}
}
}
]
}
Schema generated from the request.body
{
"$id": "http://json-schema.org/draft-04/schema#",
"$schema": "http://json-schema.org/draft-04/schema#",
"title": "Product",
"type": "object",
"properties": {
"type": {
"type": "string"
},
"features": {
"type": "array",
"items": {
"type": "object",
"properties": {
"type": {
"type": "string"
},
"geometry": {
"type": "object",
"properties": {
"type": {
"type": "string"
},
"coordinates": {
"type": "array",
"items": {
"oneOf": [
{
"type": "number"
},
{
"type": "number"
},
{
"type": "number"
},
{
"type": "number"
},
{
"type": "number"
}
],
"type": "array"
}
}
}
},
"properties": {
"type": "object",
"properties": {
"prop0": {
"type": "string"
},
"prop1": {
"type": "object",
"properties": {
"this": {
"type": "string"
}
}
}
}
}
},
"required": [
"type",
"geometry",
"properties"
]
}
}
}
}
Schema validation with Ajv
[
{
keyword: 'type',
dataPath: '.features[0].geometry.coordinates[0]',
schemaPath: '#/properties/features/items/properties/geometry/properties/coordinates/items/type',
params: { type: 'array' },
message: 'should be array'
}
]
Why Ajv detects an issue ?

Answer 1

Assuming that you want coordinates to be either an array of numbers or an array of arrays of numbers then this schema doesn't look right to me:
"coordinates": {
"type": "array",
"items": {
"oneOf": [
{
"type": "number"
},
{
"type": "number"
},
{
"type": "number"
},
{
"type": "number"
},
{
"type": "number"
}
],
"type": "array"
}
}
You would typically use oneOf to choose between different schemas but the oneOf here says:
it can be a number
or a number
or a number
or a number
or a number
But then ultimately it doesn't matter because you have a {"type": "array"} in the end that seems to override everything which would explain why it fails for {"coordinates": [102, 0.5]} because that is not an array of arrays.
It seems that what you're looking for is more along the lines of: (untested)
"coordinates": {
"type": "array",
"items": {
"oneOf": [
{ "type": "number" },
{ "type": "array", "items": { "type": "number"} }
]
}
}
Which reads:
items in a coordinates array can be either "numbers" or "arrays of numbers".
I think that your generate-schema package got this wrong here.

Answer 2

I found my answer with QuickType library