struct Product : Decodable {
    // Properties in Swift are camelCased. You can provide the key separately from the property.
    var manageStock: Bool?

    private enum CodingKeys : String, CodingKey {
        case manageStock = "manage_stock"
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        do {
            self.manageStock = try container.decodeIfPresent(Bool.self, forKey: .manageStock)
        } catch DecodingError.typeMismatch {
            // There was something for the "manage_stock" key, but it wasn't a boolean value. Try a string.
            if let string = try container.decodeIfPresent(String.self, forKey: .manageStock) {
                // Can check for "parent" specifically if you want.
                self.manageStock = false
            }
        }
    }
}