Custom `impl TryFrom` returning something other than `tokio_postgres::Error`

[sjoqvist]

I want to read aggregated information from my database in a way that is hard to represent as a table but easy to return as JSONB and, subsequently, easy to parse from JSON to a Rust struct using Serde. The naïve approach is to duplicate the entire struct with the exception of one serde_json::Value field, and then implement conversion of the entire type.

To avoid code duplication, I wanted to use #[from_row(try_from = "JsonVersionOfMyType")] and a custom impl TryFrom<JsonVersionOfMyType> for MyType. However, this requires type Error = serde_json::Error; but then I'm getting

error[E0277]: the trait bound `tokio_postgres::Error: std::convert::From<serde_json::Error>` is not satisfied
  --> crates/x/src/x/x.rs:26:35
   |
26 | #[derive(Clone, Debug, Serialize, FromRow)]
   |                                   ^^^^^^^ the trait `std::convert::From<serde_json::Error>` is not implemented for `tokio_postgres::Error`
   |
   = note: this error originates in the derive macro `FromRow` (in Nightly builds, run with -Z macro-backtrace for more info)

The problem is that there seems to be no external way to construct tokio_postgres::Error, which means that it can only be generated during communication with the database.

Am I doing something wrong? In that case, maybe you could provide sample code for how to use try_from? I've found no useful information on the Internet.

Or can this feature only be used by code that potentially returns tokio_postgres::Error when converting between two "internal" Rust types? Because that seems like a non-existent use case.

[sjoqvist]

I solved my problem by implementing postgres_types::FromSql and calling serde_json::Value::from_sql before converting to my custom struct. I wanted to skip this step as it feels redundant, but it's just one line. Since from_sql returns errors as Box<dyn core::error::Error + Sync + Send>, it's now trivial to convert to my custom struct.

Still, I'm curious about how TryFrom is supposed to handle errors. For me, it only served as a distraction.