hash_to_json_schema.rb

require "json"
require "time"
module HashToOpenApiSchema
  def self.get_type(value)
    Time.parse(value)
    "date-time"
  rescue
    case value
    when TrueClass
      "boolean"
    when FalseClass
      "boolean"
    else
      value.class.name.downcase
    end
  end

  def self.array_schema(values)
    values.map do |value|
      type = get_type(value)
      schema_main_body(type, value)
    end
  end

  def self.schema_main_body(type, example)
    {
      type: type,
      description: "DESCRIPTION GOES HERE",
      example: example
    }
  end

  def self.generic_schema(hash, output = {})
    hash.each do |key, value|
      type = get_type(value)
      case type
      when "hash"
        output[key] = { properties: generic_schema(value) }
        output[key].merge!(schema_main_body("object", value))
      when "array"
        output[key] = { items: array_schema(value) }
        output[key].merge!(schema_main_body(type, value))
      else
        output[key] = schema_main_body(type, value)
        output[key].merge!(enum: [value]) if key == "type"
      end
    end

    output
  end

  def self.run(hash)
    generic_schema = generic_schema(hash)
    json_schema = {
      required: hash.keys,
      type: "object",
      properties: generic_schema,
      example: hash

    }
  end
end

payload = {"type"=>"bookmark_with_cooksnap_comments",
 "id"=>1,
 "visited_at"=>"2021-01-28T17:11:25Z",
 "recipe"=> { title: "title", bookmark_ids: [1,"2",true] }
}

puts JSON.pretty_generate(HashToOpenApiSchema.run(payload))

result.json

{
  "required": [
    "type",
    "id",
    "visited_at",
    "recipe"
  ],
  "type": "object",
  "properties": {
    "type": {
      "type": "string",
      "description": "DESCRIPTION GOES HERE",
      "example": "bookmark_with_cooksnap_comments",
      "enum": [
        "bookmark_with_cooksnap_comments"
      ]
    },
    "id": {
      "type": "integer",
      "description": "DESCRIPTION GOES HERE",
      "example": 1
    },
    "visited_at": {
      "type": "date-time",
      "description": "DESCRIPTION GOES HERE",
      "example": "2021-01-28T17:11:25Z"
    },
    "recipe": {
      "properties": {
        "title": {
          "type": "string",
          "description": "DESCRIPTION GOES HERE",
          "example": "title"
        },
        "bookmark_ids": {
          "items": [
            {
              "type": "integer",
              "description": "DESCRIPTION GOES HERE",
              "example": 1
            },
            {
              "type": "string",
              "description": "DESCRIPTION GOES HERE",
              "example": "2"
            },
            {
              "type": "boolean",
              "description": "DESCRIPTION GOES HERE",
              "example": true
            }
          ],
          "type": "array",
          "description": "DESCRIPTION GOES HERE",
          "example": [
            1,
            "2",
            true
          ]
        }
      },
      "type": "object",
      "description": "DESCRIPTION GOES HERE",
      "example": {
        "title": "title",
        "bookmark_ids": [
          1,
          "2",
          true
        ]
      }
    }
  },
  "example": {
    "type": "bookmark_with_cooksnap_comments",
    "id": 1,
    "visited_at": "2021-01-28T17:11:25Z",
    "recipe": {
      "title": "title",
      "bookmark_ids": [
        1,
        "2",
        true
      ]
    }
  }
}