Sqlx lifetimes when the pool is generic member in a struct

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1

I have a design where storage can use different backends. I'd like to fit sqlx in that, for which I'd need to have the connection object of sqlx be a member of a struct (which will implement the storage traits later). Even though I succeeded in doing a simple query, I'm having trouble with a query that I can parse to a result. Here's a minimal example:

Both my success and failure can be seen here (unfortunately playground doesn't have sqlx, so it won't compile in there, but it compiles otherwise)

You only need these two dependencies:

sqlx = { version = "0.7", features = [ "runtime-tokio", "macros", "sqlite", "postgres" ] }
tokio = { version = "1.30", features = ["full"] }

You will see in main() a minimal example (without parametrization) that works fine. But I can't seem to parametrize the struct to do the same thing.

Here's the error once you uncomment the code in there (that I'd like to get working):

error: implementation of `FromRow` is not general enough
  --> src/main.rs:50:28
   |
50 |         let rows: (i64,) = sqlx::query_as::<_, _>("SELECT COUNT(*) as count_pet FROM misc;")
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `FromRow` is not general enough
   |
   = note: `(i64,)` must implement `FromRow<'0, <D as sqlx::Database>::Row>`, for any lifetime `'0`...
   = note: ...but it actually implements `FromRow<'1, <D as sqlx::Database>::Row>`, for some specific lifetime `'1`

error: implementation of `FromRow` is not general enough
  --> src/main.rs:50:28
   |
50 |           let rows: (i64,) = sqlx::query_as::<_, _>("SELECT COUNT(*) as count_pet FROM misc;")
   |  ____________________________^
51 | |             .fetch_one(&self.db_pool)
   | |_____________________________________^ implementation of `FromRow` is not general enough
   |
   = note: `(i64,)` must implement `FromRow<'0, <D as sqlx::Database>::Row>`, for any lifetime `'0`...
   = note: ...but it actually implements `FromRow<'1, <D as sqlx::Database>::Row>`, for some specific lifetime `'1`

error: lifetime may not live long enough
  --> src/main.rs:50:28
   |
41 |     pub async fn is_initialized<'e, O>(&'e self) -> Result<bool, Error>
   |                                 -- lifetime `'e` defined here
...
50 |         let rows: (i64,) = sqlx::query_as::<_, _>("SELECT COUNT(*) as count_pet FROM misc;")
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'e` must outlive `'static`

error: could not compile `sqlx_generics` (bin "sqlx_generics") due to 3 previous errors

What I don't understand is: Why does sqlx care about the lifetime of the raw &str I pass there... I tried all kinds of games but nothing works. I would really appreciate assistance in getting this to compile.

Also if you have hints on how to do better with sqlx I do welcome your suggestions. I feel like I'm trying hard, not smart.

2

That's not the problematic lifetime (it is 'static after all), 'e, the lifetime of the reference to self with the database pool is the problem. I've changed your where clause a little bit to make your bounds not dependent on the concrete lifetime of &self, but instead use HRTBs to express that these bounds should be applicable to any lifetime 'e and not the concrete lifetime of &self. Makes it compile on the Rustexplorer[1].

  1. Where you can import and use sqlx, but only till version 0.6. ↩ī¸Ž

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3

This is awesome. Thank you very much! Even though I've been doing rust for years, I still have to learn more and more on how to do lifetimes properly. Cheers!

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