How do I perform additional database queries when serializing a value with Serde?

Question

A Program has a one-to-many relationship with Project. I'm looking for an ideal way to include all the projects in a program when I print the JSON response.

extern crate rocket_contrib; #[macro_use] extern crate serde; #[macro_use] extern crate serde_derive; #[macro_use] extern crate serde_json; use rocket_contrib::Json; use serde_json::Value; mod schema { table! { projects (id) { id -> Int4, title -> Varchar, program_id -> Int4, is_archived -> Bool, } } table! { programs (id) { id -> Int4, title -> Varchar, is_archived -> Bool, } } } fn main() { let program = Program::get(id); let json_value = Json(json!({ "result": program })); }

I looked into implementing custom serializing:

impl Serialize for Program { fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: Serializer, { } }

But I do not have the db::Conn required to pull the projects in this function.

I'm looking for a response like:

{ "id": 1, "is_archived": false, "title": "Program 1", "projects": [{ "id": 2, "is_archived": false, "title": "Project 1" }] }

I think you need to define a struct ProgramWithProjects or similar, representing the desired structure, including a field projects: Vec<Project>. You can fill the struct while you hold the database connection, and then serialize it to JSON. — Sven Marnach
– Sven Marnach, Commented Oct 26, 2018 at 19:36

Shepmaster · Accepted Answer · 2018-10-26 23:48:53Z

The practical answer is don't. Serialization of a value should not involve arbitrary database calls, it should only serialize. Create a struct to hold all the data:

#[derive(Serialize)] struct ProgramAndProjects { #[serde(flatten)] program: Program, projects: Vec<Project>, }

Then write a function that populates that struct by doing the required database queries and directly serialize the result.

This has a huge benefit of being wildly more testable, as well.

That being said, it is possible to do, but it's likely not worth it. You can stick a database connection into Program (or create a newtype with the connection and the Program) and then make use of the connection during serialization.

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