final class Post: Model, Codable {
    // other stuff...

    // MARK: Codable compliance
    private enum CodingKeys: String, CodingKey {
        case id
        case content
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        // Note that Identifier is not Codable, so as a workaround, I'm always representing it as a String.
        try container.encode(id?.string, forKey: .id)
        try container.encode(content, forKey: .content)
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        content = try container.decode(String.self, forKey: .content)
    }
}

// You can now use request.decodeJSONBody(Post.self), post.makeResponse(), etc.