Expose multi-db connection inference from diesel-cli

[igalic]

Setup

in Plume and Rustodon we want to support PostgreSQL and SQLite for smaller or dev/test setups.
Both projects are currently working on patches adding SQLite support: Plume-org/Plume#226 rustodon/rustodon#12

Versions

• Rust: nightly
• Diesel: 1.3.×
• Database: postgres & sqlite
• Operating System

Feature Flags

[dependencies.diesel_cli]
features = ["postgres", "sqlite"]
version = "1.3.1"

[dependencies.diesel]
features = ["postgres", "sqlite", "r2d2", "chrono"]
version = "*"

Problem Description

our problem is that we have to reimplement a lot of diesel-cli functionality for the setup and the normal operation of the program if you want to support more than one database.

currently, this seems to be in

diesel/diesel_cli/src/database.rs

Lines 75 to 86 in db9ffe2

	pub enum InferConnection {
	#[cfg(feature = "postgres")]
	Pg(PgConnection),
	#[cfg(feature = "sqlite")]
	Sqlite(SqliteConnection),
	#[cfg(feature = "mysql")]
	Mysql(MysqlConnection),
	}
	impl InferConnection {
	pub fn establish(database_url: &str) -> DatabaseResult<Self> {
	match Backend::for_url(database_url) {

but i'm not sure how to acces or if that's possible at all

What are you trying to accomplish?

We would like to easily use both databases transparently

Checklist

• I have already looked over the issue tracker for similar issues.

this might be related to #357

• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)

[sgrif]

Thanks for the feature request. I can see why pulling this out of Diesel CLI might be useful for your use case. However, there's a big difference from both a design and maintenance point of view between something hacked together for internal use, vs something we're comfortable supporting as a stable API designed for public consumption.

Hopefully it's clear why providing a public API for this is a lot more work than "just making this code public".

While multi-db support is possible with Diesel, it's not a primary goal of the project. For that reason, developing a feature like this simply isn't on the roadmap in the near future. That said, if the code in Diesel CLI today suits your need, and you feel it's acceptable to maintain as a public API, I'd encourage you to take it and release it as your own crate. Diesel might even end up depending on it.

However, I don't see this feature getting implemented as a standalone library any time soon.

[igalic]

Thank you very much for this reply — it does help put things into perspective.